I first noticed this with the end of November firmware updates, initial with my WiiM Amp:
Firmware 5.0.636939 and WHA 3.0.3 now and yes, saving independent L/R room correction to a single preset is working now.
However, I do experience quite a number of oddities.
In the Room Correction Settings the Subwoofer Calibration toggle switch is available even if the subwoofer output is disabled. Wouldn't it be less confusing if the option was greyed out then? Not so much of an issue and finally a matter of taste.
The Acoustic Measurement screen does not show the microphone in use when performing independent left/right correction. It does in stereo correction mode...
I thought it might be rather related to the WiiM Home app version than the firmware version and at some point in time there have been reports as if WHA 3.0.4 had solved the issue.
Now, either it's a regression bug in WHA 3.0.7 or it's never been really fixed. I cannot tell. However, the external mic
is used, even if not shown in the app.