I'm using a mod on a modified mohist 1.20.1 server core and I encountered a problem after installing the mod. Items at random times cannot stack with each other, the problem was solved after deleting the mod. I don't know much about this, but if you need any additional information I will provide it.

i faced same issue.
Thats what i found when checked item data using /data command

{Slot: 13b, ForgeCaps: {"celestisynth:celestisynthitemstackcapabilities": {cs.aquaSkin: 0}}, id: "ars_nouveau:red_archwood_log", Count: 1b}
{Slot: 6b, ForgeCaps: {"celestisynth:celestisynthitemstackcapabilities": {cs.aquaSkin: 0}}, id: "ars_nouveau:red_archwood_log", Count: 63b, tag: {}}
{Slot: 19b, ForgeCaps: {"celestisynth:celestisynthitemstackcapabilities": {cs.aquaSkin: 0}}, id: "ars_nouveau:red_archwood_log", Count: 18b, tag: {}}

You can see that every "stack" has "celestisynth:celestisynthitemstackcapabilities"

Found the issue. Will be fixed soon.