Today we have a bug in the extension for backpacks
after installation all 8 rows of the backpack are available. as soon as you then use the XPStats and set the first point everything works but if you then reset the points you have all 8 rows again

are you assigning permissions in the backpacks mod? Make sure you do not set any permissions for backpacks if so you must remove all permissions and let XPerience control the permissions itself