I do not know. It is not a plugin I often use. Truthfully, still using Gimp 2.10 a lot of the time 
Do you have an example xcf with typical layers / layer groups. If you save as a Gimp 3 compressed filename.xcf.gz it should attach. Perhaps prune the 30 layers down a bit first.
EDIT: Do you have layer groups ? Maybe try changing Output to "Single Directory"
Do you have an example xcf with typical layers / layer groups. If you save as a Gimp 3 compressed filename.xcf.gz it should attach. Perhaps prune the 30 layers down a bit first.
EDIT: Do you have layer groups ? Maybe try changing Output to "Single Directory"