Both developer and public beta are available. Not a myth.
If you have installed the 10.15.5 supplemental update you will not be offered 10.15.6beta1 which does not have the security fix included in the supplemental. I had NOT installed the supplemental and was offered the beta.
In your case, best to just wait for the next beta.
Same problem here. I decided not to roll back via Time Machine so a bit stuck now. I'd hoped I could just use recovery to reinstall the pre-supplemental version, but the app store catalina installer is already updated -- and I don't have a ful ldev account (I assume the images are avail there)
So like others a bit frustratingly stuck. You'd have thought apple would have know this would happen and deferred or re-issued the beta. Maybe they will in a few days.. or may just have to wait for beta 2.