I can sort of confirm this behaviour now.
My WiiM Ultra is on firmware 5.2.626261 and the WiiM Home App version is 2.11.1.240813 for Android.
My perception is that the pre-gain setting actually does get stored (I didn't try a full power circle), but the display doesn't reflect this. All values always display 0 when you enter the pre-gain settings again. That's pretty much unusable, because you never know the current setting. Touching the slider bar anywhere will just switch to that actual value, so you pretty much have no control.
This leads me to the next problem, the slider. There is really no need for 0.1 dB resolution here. 0.5 dB or even 1 dB would be fine, but please give me plus and minus buttons in addition to the slider. Make it e.g. like setting the subwoofer level,.just limited to +/-10 dB.