Not sure what is happening, but I'm unable to create a Soul Shard in singleplayer (have not tested multiplayer). Here's my modlist:

architectury-4.4.68.jar
soulshards-1.18.2-1.2.2.jar
polylib-forge-1801.0.2-build.13.jar

The pattern I tried (and using normal quartz blocks too, this is the picture with quartz pillars):

I tried downgrading Architectury a few versions, that didn't work too. I also tried to make a multiblock.json file in the root of a datapack (data/multiblock.json) as seen in the files of this repo, but it didn't work either. I tried standing on top and inside the glowstone block as well (using pistons).

I am right clicking the glowstone block in the picture with a diamond
Forge 40.1.25
Minecraft 1.18.2

Works for me in dev/test instance with just it + deps installed and in the Forgecraft instance.
https://ss.gigabit101.net/javaw_lgM5pU8Oj7.gif

Do I have to use Smooth Quartz? I'll retry again with it in a bit

You need to use Block of Quartz

Tried with block of quartz and it works now, apologies for the report and thanks for your help!