I have the rare backpack. But once I put items in that would fill the bag up past 33 item slots, everything past that kicks back into my inventory the next time that I open the backpack. Also, when I click and drag items into the backpack, no matter how many items are in it, they just shoot out onto the ground when I close the backpack. This doesn't happen when I shift click to input items.

what version of the mod?

Playing on the FTB Endeavor pack 1.9.0 and the version of simplybackpacks is 1.16.5-2.0.1-build.27

im assuming you are opening the backpack via hotkey and not in hand?

Correct. I just tried it with the backpack in hand and it seemed to work fine. I can't recreate the issue right now, but it has been somewhat sporadic. Others on the server have noticed the same thing with their various sized backpacks though.

so, that bug was fixed in 2.0.2, the workaround for 2.0.1 is to open it in hand at least once after upgrading it, as opening it finishes the upgrade.