I can absolutely understand why this could feel frustrating after we began to enforce the rule that a guest must have a different email domain.
The idea for Guests is that they are meant to be people who are external to your account, such as clients, freelancers, etc., which internal coworkers would not fit into the category of.
While you may have added your account before we began enforcing the rule, it was technically still in the requirements for a guest to need a different email domain, which is why that access was lost after we began to enforce it.
If you would like, you are free to open a feedback post showing support for same domain guest users! - Feature Feedback Page
While I know this is not what you were maybe hoping to hear, I hope this helps!
Cheers,
Jenna