Version Info

• Minecraft, 1.20
• Packed Up, 1.0.28 Fabric
• Core lib, 1.1.9a
• Config lib, 1.1.6

Description of the Bug
I have disabled all backpacks except for the basic. I have set the rows to 9 and the columns to 13, which are stated as the upper limit for each setting in the config file. I also configured the option to keep the bag on death.

Steps to Reproduce
Restart server and give the backpack to a player. Each time the backpack is created with the default inventory size. Have not been able to test whether or not the bag is kept on death.

Screenshots
...

Seems to be working fine with the versions you mentioned. Keep in mind that the server needs to be restarted when you change any of the config options.
Apart from that I have no clue what could be wrong. When I set the size to 9 rows and 13 columns the backpack indeed has that many slots when I open it on the server.