When pasting a schematic from 1.20.2 or older with a container containing a grass item in 1.20.3 or later the grass item will be deleted.
The cause of this issue is that grass got renamed to short_grass in 1.20.3. And there doesn't exist a grass item anymore after 1.20.3 so it will get deleted.

There is currently no item conversion in inventories, and the block conversion support between MC versions is also very minimal and basic.

I have a very old work-in-progress branch where I was working on better schematic conversion support between MC versions, but it was left kinda half way done and I haven't had time to work on it since I started working on the client mods rewrite stuff back then... So a fix for this will have to wait for that work to continue at some point. Otherwise it would just be another temporary special case hack fix...