I am struggling with the same issue.

Also with Rasa 3.6.2.

I have multiple different types of slots. After the first few slots, the form is interrupted with my custom fallback action.

Particularly, I see that:

rasa.core.policies.memoization  - There is no memorised next action

Then it finds that the fallback should occur. I do not understand why it goes into the AugmentedMemoization Policy in the first place, if I have specified the form in a rule already.

Have you found a working solution to your problem?