FW version 7.55.20220718 (released on 2022-07-29). Same behavior was noticed after "Default Setup", also with various analog channel combinations. Not every time other channel traces are shown on screen, but left side axis goes through other colors every time. This is not a bug IMO.
It seems that "Push For 50%" functionality is a subset of "Auto Scale" functionality. Presumably you want the scope to setup itself based on input signals, so "Push For 50%" configures all analog channels triggers to middle of the waveform (so that if trigger source is changed, scope still triggers), irrespective if other channels are turned on or off.
KS manual
should read:
You can push the Trigger Level knob to preset the level of all analog channels to the waveform's 50% value.