It worked two weeks ago without any issue. Not sure what changed but it no longer works in my production bot. I tested the regular adaptive card flow and it works with no issue - only if you wait for a response it fails. Also, posting as Flow Bot works as well. May I know if the PVA team broke this feature somehow?
You are right, there is a bug that got introduced. The good news is that we have a fix and it's being deployed now. It will take a few weeks to fully deploy world-wide. It's hard to predict exactly when the deployment will finish -- there are many contingencies. But it's going on now.
I suggest just using Bot Framework Composer to send the adaptive card if you are using premium PVA. Gave up on this flow action - it was laggy in the past and now straight up doesn't work.