Photos are taken using preview resolution / unreliable mode setting
Despite of:
capture-width=4208
capture-height=3120
capture-rate=30
capture-fmt=GRBG8
preview-width=1052
preview-height=780
preview-rate=30
preview-fmt=GRBG8
in the config, pictures are taken in 1052x780 resolution, which is a significant downgrade in image quality over older Megapixels build.
Millipixels sure tries to change the mode - you can hear the focus lens working - but it doesn't manage to. Also, when the resolution is changed externally by a different app (like the older media-API-but-pre-libcamera build of Megapixels), the Millipixels preview stays at the resolution set by that external app.