After recent update, the backpacks permissions are not working.

Can you check that?

All players lost their permissions after update

solved disabling it:

"Dynamic Size (EXPERIMENTAL)": {
"Enabled": false,

I don't know why it appeared "true" after update

Can you clarify what you mean by the players lost their permissions? Do you mean that the permissions still appear (in a permissions manager plugin, for example) but they are not granted to the players and groups as they were before? If so, were you able to grant the permissions again and did that resolve the issue?

This plugin does not grant or revoke permissions, so if players lost permissions, that would have to be caused by something else, such as a malfunction of the framework.

Edit: Looks like you already replied since I was writing my response. I did not change the default value for dynamic size. I checked and it is still set to false.

Yes, I did resolve the issue. After update, the permissions were not working because Dynamic Size was "True" and If it was true before it was not working on previous versions, I don't know. 
But everything is working great! Thank you for your reply :D