I haven't seen this.
I added my wife to Family Sharing on day one, but she wasn't actually activated till a month and a half later.
If your having this problem, I think the only logical option is to update her phone and accept the invitation.
Beta's are great, but they are hairy territory. If you aren't ready to bring everything you have to the same development level then don't do it.
I know it's trite, but it's true.
AppleTV for instance is beautiful in it's development iteration, but man it doesn't work well with most iTunes Extras. Otherwise it's fine.