Make sure the exec permission is the last one in your fstab entry.
What your describing is one of the many ways my issue manifested.: Automount not providing exec despite fstab flag and user perms?
Steam tries to run a script on every mounted device to check for appropriate privileges. If it doesn’t have exec it won’t present the library as a valid option, meanwhile If exec is not the last option in your fstab it can be overridden by some other option.