Yep they probably won't even issue you a boarding pass since the Thailand Health Pass form is a mandatory requirement for boarding a flight from those countries. Airlines that don't comply could be fined.
It's a certainty to be denied entry and in fact, you need to fill out the T.8 health form with your yellow fever vaccination certificate uploaded just to get issued a boarding pass. This requirement was imposed last August in the wake of the monkeypox 2.0 "outbreak".
Anonymous participant 959 Yes, correct. One individual down in Krabi claims otherwise, but I told her she's wrong and a "know it all". Usually, you'll receive a rejection email after the initial filing, something she probably isn't aware of.
I don't know why they created this bureaucratic nonsense, although at least you can always mail your 90 day report in rather than going in person, regardless of whether it's your first or 10th time following your last re-entry.