If a user is deactivated, which means it belongs to no existing group in JIRA, it is still able to be set as an issue watcher if it has the Browser Project Permission.

Steps to replicate:

1. deactivate an user.
2. try to add it as watcher in any existing issue

Result:

User is listed as a possible watcher, but if you select them an error comes up saying that you can't add them as a watcher

Expected Result:

User should not be listed.