I was using the latest version of Spell Engine and Spell Power Attribute for 1.19.2 and noticed that items the player gets (like from the creative inventory) often have an empty nbt tag (that can be seen with /data get entity @s SelectedItem), while items that are generated in chests dont have it, causing said items to not stack when in survival mode; for some reason they still stack in creative. Disabling those 2 mods solved the issue.

seems to happen on both 0.9.3 and 0.9.5

Hello!
Thanks a lot for the report and the all details.
The command you gave helped a lot too.
If you have a bit of spare time feel free to join Discord where I can give testable versions.

Fixed in 0.9.6.
Please reopen if the fix was unsuccessful, or the issue resurfaces.