The bug
If an entity is standing on soul sand or mud, and this soul sand/mud block is converted to a sculk block by a sculk catalyst, any entity standing on a converted block will fall through it.
How to reproduce
Create a platform of soul sand
Place a sculk catalyst on this platform
Surround the catalyst with armor stands
Kill a mob near the catalyst
Expected results
Any armor stands atop converted blocks would be pushed upwards slightly as to accommodate the new block's hitbox.
Actual results
Any such armor stands clip inside of the block and fall.
Further notes
While fixing this, I'd recommend also testing this with data packs so that other blocks with non-full-cube hitboxes (e.g. farmland, grass path, slabs) are also fixed if added to the sculk convertibility tag.
Related issues
Attachments
Comments

I can also confirm this behavior. Similar to MC-250034, this could be fixed by calling the pushEntitiesUp()
method where appropriate, just like how farmland does when it converts to dirt to avoid entities standing on top of it from "falling" into the ground.