Curved tracks have been historically problematic in KiCad.
The simple answer is that it’s just not supported in KiCad V5.
There are some complicated workarounds, you can for example use graphic arcs and place them on a copper layer.
This is a good thread to read for such things:
It’s about a flex PCB to be mounted in a camera lens, and it needs lots of curved tracks. I tried to help finding workarounds back then.
The issue is being worked on actively though.
KiCad-nightly V5.99 has several different ways to add curves to copper tracks. At the moment there are probably changes made to that code daily or weekly. It’s probably one of the reasons that KiCad V6 is delayed.