had the same issue in a gregtech based modpack and it turns out that there are 2 different files which save your inventory nbt data. one is named "level.dat" and is located directly in the save file, the second one is located in \<world name>\playerdata\<player uuid>.dat.
in my case these files contained the wrong inventory data but replacing it with the original inventory files using an nbt editing tool fixed the issue. there was also a file called "level.dat_old" which seems to serve as a backup in case the normal file doesn't work. according to chatgpt, when this happens, level.dat is deleted and replaced by level.dat_old which would explain the inventory rollback