We have two guest accounts, both people from the same external organization. Lets call them Jane Doe and John Smith from contoso.com. If I open a new chat and type Jane Doe, Teams finds her guest account and offers Jane Doe (Guest) as the match for the name, and all works fine. If I type John Smith, it does not find him. If I enter John Smith's email address johns(at)contoso.com it is able to open a chat, but lists him as John Smith (External) and of course the options for the chat are restricted (can't add a new tab for example). I do not see any settings differences between the two accounts, both guest accounts were created by the invitation method. The biggest difference is that Jane's account is about 4 yrs old, where John's is only 1 week old.
