Invited guest collaborators being turned into full members automatically

Hi,
Recently we noticed that invited guest collaborators were being automatically turned into managed users and taking up seats.

This appears to be a relatively new problem for us as there are guest users in our database with organisation domain email addresses on their accounts - they are not managed users.

1. Is there a new setting or new enforcement in place?
2. Anything we can do to stop this happening?

We ideally would be able to add volunteer users as guest collaborators even if they are invited via their temporary organisation domain email address.

I hope my explanation of the problem is clear enough.

Thanks

Tim - Senior Developer for REACT Disaster Response