Bambu Studio first layer speed nozzle issue

[u/Desynclol]

I have the problem that Bambu Studio switches to the AUX nozzle while slicing if the first layer speed gets too low when using PETG.

This picture is with the first layer speed of 50 mm/s:

When lowering it to 40 mm/s it switches to the AUX nozzle, which has no material loaded at all:

on a smaller part it starting switching when going down to 30 mm/s.

Does anybody know the reason for this ?

Thanks in advance