I found some incorrect block IDs in the overlays. Some of the things listed might of course be me missing some sort of functionality, but to my understanding only actual block ID's used by Minecraft work here. I have of course tried to see if incorrect ID's work in-game anyway and they do not (at least for me).

List by affected connectBlocks:

1. basalt

   • dirt is listed twice
   • grass_path is listed twice
   • grass_path should be dirt_path
   • silver_glazed_terracotta does not exist
   • sandstone_stairs do exist, but cause clipping; additionally, no other stairs are mentioned in basalt

2. bricks

   • concrete does not exist
   • terracotta is listed twice

3. calcite

   • grass_path should be dirt_path
   • hardened_clay does not exist
   • planks does not exist
   • silver_glazed_terracotta does not exist
   • stained_hardened_clay does not exist

4. coarse_dirt

   • the line starting with concrete= does not seem to do anything
   • grass_path does not exist (dirt_path is already included)

5. crimson_nylium

   • dirt is listed twice
   • grass_path should be dirt_path
   • hardened_clay does not exist
   • planks does not exist
   • silver_glazed_terracotta does not exist
   • stained_hardened_clay does not exist

6. diorite

   • pointed_dripstone does not seem to do anything

7. rooted_dirt

   • grass_path should be dirt_path
   • hardened_clay does not exist
   • planks does not exist
   • stained_hardened_clay does not exist

8. grass_block

   • hardened_clay does not exist
   • planks does not exist
   • silver_glazed_terracotta does not exist
   • stained_hardened_clay does not exist

9. gravel

• grass_path should be dirt_path
• hardened_clay does not exist
• planks does not exist
• silver_glazed_terracotta does not exist
• stained_hardened_clay does not exist

11. hay_block

    • grass_path should be dirt_path
    • hardened_clay does not exist
    • planks does not exist
    • silver_glazed_terracotta does not exist
    • stained_hardened_clay does not exist

12. moss / moss_block

    • grass_path should be dirt_path
    • hardened_clay does not exist
    • planks does not exist
    • stained_hardened_clay does not exist

13. mud

    • does not have a .properties file

14. mycelium

    • dirt is listed twice
    • grass_path should be dirt_path
    • hardened_clay does not exist
    • planks does not exist
    • silver_glazed_terracotta does not exist
    • stained_hardened_clay does not exist

15. packed_mud

    • grass_path should be dirt_path
    • hardened_clay does not exist
    • planks does not exist
    • stained_hardened_clay does not exist

16. podzol

    • dirt is listed twice
    • cobblestone_mossy_beach does not exist
    • cobblestone_mossy_birch does not exist
    • cobblestone_mossy_coldbeach does not exist
    • cobblestone_mossy_darkoak does not exist
    • cobblestone_mossy_desert does not exist
    • cobblestone_mossy_extremehills does not exist
    • cobblestone_mossy_forest does not exist
    • cobblestone_mossy_iceplains does not exist
    • cobblestone_mossy_jungle does not exist
    • cobblestone_mossy_mushroom does not exist
    • cobblestone_mossy_plains does not exist
    • cobblestone_mossy_swamp does not exist
    • cobblestone_mossy_taiga does not exist
    • concrete does not exist
    • grass_path should be dirt_path
    • hardened_clay does not exist
    • planks does not exist
    • podzol does not seem to do anything (same block)
    • silver_glazed_terracotta does not exist
    • stained_hardened_clay does not exist
    • stonebrick does not exist (stone_bricks is already included)
    • cyan_terracotta is listed twice
    • dirt is listed twice
    • grass_path is listed twice
    • grass_path should be dirt_path
    • sandstone_stairs do exist, but cause clipping; additionally, no other stairs are mentioned in red_sand
    • silver_glazed_terracotta does not exist

17. reinforced_deepslate

    • does not have a .properties file

18. sand

    • dirt is listed twice
    • grass_path is listed twice
    • grass_path should be dirt_path
    • silver_glazed_terracotta does not exist

19. warped_nylium

    • dirt is listed twice
    • grass_path should be dirt_path
    • hardened_clay does not exist
    • planks does not exist
    • silver_glazed_terracotta does not exist
    • stained_hardened_clay does not exist

20. amethyst_block / budding_amethyst

    • dirt is listed twice
    • grass_path should be dirt_path
    • hardened_clay does not exist
    • planks does not exist
    • silver_glazed_terracotta does not exist
    • stained_hardened_clay does not exist

21. deepslate / deepslate_*_ore

    • 1.properties has empty matchTiles=
    • 20.properties has empty matchTiles=

22. dirt / rooted_dirt

    • grass_path should be dirt_path
    • hardened_clay does not exist
    • planks does not exist
    • stained_hardened_clay does not exist

I had a plan to programmatically create my overlays but I had some work to do to parse every files I have currently. This should be really powerful et mitigate any current and upcoming issues. For your information, every texture/blocks mentioned are blocks that were present at a point. These are likely gonna stay to keep being compatible with older versions.

That makes sense to me. I'll try and fix the other stuff myself and submit a pull request (i have no idea how that works lol, but I'll figure it out)

Noticed a bunch of connected textures (mainly grass) weren't working, but only with Sodium for some reason @xalixilax

Though I suppose some overlays not working with sodium could be a separate issue?

Though I suppose some overlays not working with sodium could be a separate issue?

yep, thats a separate issue. if you open this as a new issue you should ping/mention me or link it here, i could say some stuff thatd be off topic for this issue