Steps to reproduce:
1. Clone a repository from remote (eg. Bitbucket) as a normal local repository.
2. Clone that same remote repository into a super-project as a submodule.
3. Open up the submodule in a new tab

• Also ensure that SourceTree checks remote for updates every x minute.
  4. Make a new commit in remote repository.
  5. Observe the results

Results:

#1 The local repository tab immediately notified the changes in the remote repository.

#2 The super-project tab notified that the remote has some changes in less than a minute or so.

#3 The submodule tab sort-of notified the changes after a few minutes, but it couldn't trace the changes made - i.e. the Pull button at the top notifying the number of changes made in remote without actually showing what changes - unlike the local repository tab. (Unless I click F5 to refresh, then it'll show).

Conclusion:
The submodule tab could not show the changes done despite knowing that there is changes made in remote, and even that takes quite a while to notify.