Just an aside, but it may help others who have this issue to share the solution that worked for you

Ooh what a great idea! I am definitely stealing this method, right down to whoever Kevin is!

I’d recommend reaching out to YNAB support. They can even view your budget (if you give them permission) and determine if it is a bug or something is configured wrong.

Hmm I don’t think it should. Have you checked there are no negative balances in any of your categories?