steps to recreate:

1. create a folder
2. enable folder based permissions
3. add a user and grant all the available permissions
4. create a multibranch job in the folder
5. in branch source, choose gitlab.
6. user gets the following error message between the "projects" section and the "Behaviours" section:
   ------------------------------
   Access Denied
   <username> is missing the Job/Build permission
   --------------------------------

workaround:

granting the user the Job/Build permission in "Configure Global Security " solves the problem, but this is major breach in security.

it appears as if the plugin doesn't take into account the permissions granted to the user at the folder level.