While I was doing a implementation in a customer, I faced a weird thing, I created a new Site Collection, and when I was trying to invite people that I had in the B2B tenant, I was not able to find them.
If I tried with an internal users, I was able to find them, so I was sure that the problem was with the external users.
The first thing that I checked, was the properties of the SC to be sure that I can invite external users, and of course I checked this parameter at tenant level, was with no luck.
So, I started to check commands in Docs, and I found a command very interesting: ShowpeoplepickerSuggestionForGuestUsers
So I decided to give it a shot:
Once executed the last command, I was able to find guest users