@Hannes7 maybe it was a glitch of the system. I still have a pending request in my inbox for two months now, while the booking was confirmed and I have hosted the guest. I asked Airbnb team and they told me it was a glitch and I should not worry about it.
Maybe something like that caused you the need to accept after having pre-approved.
I don't think that it could be related to the guest's reliability. I mean I haven't read something relevant among Airbnb instructions about requests.
By the way, your listings look amazing!