Insulation of the floor ceiling: Can it stay like this?

The gap actually creates additional safety. It would have been even better to use a diffusion-open material for the flooring / attic. If I understand the construction correctly, the vapor barrier in the attic could also have been omitted. I assume you have an airtight floor retractable ladder.

Yes, exactly. We decided to insulate both (the entire roof slope up to the ridge & the ceiling). because the attic may later be partially converted. At least conceivable as a playroom, office, or similar.

Heating would then have to be done electrically there, since we have not installed and will not install underfloor heating in the attic.

How does the situation change if we were to heat in the attic? The gap would then have to be concealed behind drywall or similar.