Cause: Skype Coexistence ModesIn my test scenario I switched my tenant over from Teams Only to Skype Only and waited overnight, and I woke up to a missing chat tab with my guest user. So we definitely have a root cause for the issue. The question is it by design? I Expect the other two Skype coexistence modes to also effect chat for guests.
Work Around: Teams Only Coexistence ModeA work around would be to set your users that should be Skype Only manually to Skype Only coexistence mode per user. Once this is set, I personally would give it some time to bake. Next switch your Org setting to Teams Only. This should now revert the guest user to be able to chat and maintain your users upgrade mode.
The side effect to this method is your new users will be provisioned into Teams Only mode. So you would have to remember to change their coexistence mode when creating their accounts. This could also cause delays in the message routing as well so you will want to plan accordingly.
So far I haven't received any feedback on this item but I do not think it's by design. Not sure if it will get fixed or not, but right now the work around is the only method for allowing guests to have chat. This seems to have started appearing recently since the updates to the Coexistence Modes starting to enforce their intended behaviors (Disabling chat etc.).
If I get anymore information about this I will update this blog.