BUG nlu fallback instead of correct respond

during the conversational test, I found the following bug on rasa X instead of giving the right response:

utter_bot_challenge(0.98)

RASA X give:

action_default_fallback(0.30)
rewind
action_listen(1.0)

after reloading the page - it works fine.

I am trying to understand what is the reason of that behavior and how to fix that.

Server:

4 Core 16GiB Ram 150GB SSD

Rasa X: 0.39.2 Rasa: 2.6.0-full