Minecraft version
1.16.3 connecting to 1.12.2 server

Mod version and malilib version
litematica-fabric-1.16.3-0.0.0-dev.20200920.161640
malilib-fabric-1.16.3-0.10.0-dev.21+arne.1

Description of the bug
Using multiconnect-1.3.28 and connecting to a 1.12.2 server using a 1.16.3 client some blocks that are in previous versions are not loading correctly. As you can see in the screenshots the slime blocks are not loading in even though they are present in both versions. Not too sure if this is a fixable issue or if its just wishful thinking.

Screenshots

Are you loading a Litematica schematic that was made in 1.12.x? There is no full/proper cross-MC-version conversion support yet for loading schematics, except for the support for loading old MCEdit/Schematica format *.schematic files in 1.13.2+.

If you actually want to use a 1.16.3 client when playing on a 1.12.2 server, then for now you should convert the 1.12.x schematics to 1.16+ either by

• Pasting them into a temporary world in 1.12 and then loading that world in 1.16.3 and saving it again from that
• Export it to the *.schematic format in the 1.12.2 version of Litematica and load/Import that in the 1.16.3 version
• Use the temporary/WIP version of Litematica on 1.12.2 that has preliminary support for cross-version conversions, to directly convert and save it in the 1.15.2 format (not sure if it works correctly works this way, I've mostly converted stuff 1.15.2 -> 1.12.2 thus far).

All the in-mod conversions/importing/exporting are done via the Schematic Manager menu.

That temporary version with preliminary cross-version conversion support is available here:
https://masa.dy.fi/tmp/minecraft/mods/misc_test_versions/malilib-1.12.2-0.10.0-dev.30.litemod
https://masa.dy.fi/tmp/minecraft/mods/misc_test_versions/litematica-1.12.2-0.0.0-dev.schematic_conversion_wip1_2020_06_02.litemod

Don't use that WIP version for other "normal use" as it has tons of schematic rewrites and there may be unknown bugs in other things, and also that dev.30 version of malilib won't even work with any released version of MiniHUD...

And finally, if you don't have a good reason for using a 1.16.3 client when playing on 1.12.2 server, I would recommend just using a 1.12.2 client with the LiteLoader versions of my mods. They have lots of new features and improvements compared to any of the current 1.13.2+ versions. Plus the next round of major updates with tons of usability updates will also come to 1.12.2 first.

The 1.12.2 versions are available here: https://masa.dy.fi/mcmods/client_mods/?mcver=1.12.2

I'm dumb. This is caused by the change to how items are handled if earlier versions with block data values and the names of blocks. In my particular case this litematic was saved using minecraft:slime_blocks but in 1.12 the block is called minecraft:slime.

This leads into my second point, (if you want me to make a specific feature request I can and I understand you're currently busy working on other thing) would you be able to add the ability to substitute blocks for other blocks? For example switch out minecraft:slime_block for minecraft:slime. I think this would have other applications as well.

You have been able to change blocks in the schematics for a long time already, using the "Rebuild" mode. There is a wiki article about it here https://github.com/maruohon/litematica/wiki/schematic_editing

However that mode will not help you with this issue, where the schematic has blocks that don't exist (by that name at least) in the current game version, as they will turn to air when the schematic is loaded. For the vanilla 1.13 flattening changes, there will be support for handling those automatically. I worked on that a few months back and it's partially working, but not yet finished (see the previous message). So it will become possible to load 1.12 schematics in 1.13.2+ or loading 1.13+ schematics in 1.12 (any new blocks will vanish in this case though) once all that stuff is finished.