Hello, me again upgrading barrels and messing everything up..

This time, just created Limited Barrels IV, placed, filled with wool, taped, destroyed and put in their place. Then, a Basic to Diamond upgrade is applied to them, which changes their color (close to acacia, Idk.. they were oak), all item icons are missing (though I can still click to get one, and if piped, an item can go inside).. But the barrel cant be accessed, as it causes a crash.

Link to crash log: https://gnomebot.dev/paste/mclogs/h8JVFIL
Link to normal minecraft log: https://mclo.gs/knIynMa

After reloading the game, upon entering the world, the barrels seem to be working now

I was just able to recreate, there's a bit of code that updates legacy barrel counts and fill levels and looks like this code was run at incorrect time and broke barrels the way you described. Just fixed that in 1.21 and fixed release will be up on curseforge / modrinth shortly.