Facebook app testers not receiving notification

I am writing an Android app which makes use the the Facebook SDK (specifically the login and friends list). I added the first 4 testers to https://developers.facebook.com/apps/{APP-ID}/roles/roles/ without any problems (these users were not asked to confirm that they would like to be testers they were just made testers immediately) and since then any tester I add is marked as "pending" but the user never receives a notification inviting them to become a tester (except for one instance where a user did receive a notification in their normal Facebook notifications). I believe this question has been asked before but so far there is no answer and Facebook provides no forum to discuss this problem in, evidenced by this question https://www.facebook.com/help/community/question/?id=3608173820208

Well as I mentioned the first couple of developers I added were added immediately (and these were users who also did not have Facebook Developer accounts), and after that any friend I added was immediately marked as "Pending".

Since then I found that:

a) Users will only receive the notification inviting them to become a developer on a desktop or laptop (not a mobile device)

b) When a user does accept the invitation the confirm button will remain disabled until the user signs up for a Facebook Developer account.