Ok pinned the problem : it is the boot.cmd/boot.scr which is NOT updated with a straight upgrade from 5.20 to >=5.25.
Let's resume this :
- if you load the original boot.cmd from 5.20, gc2035/vfe_v4l4 simply work.
- if you load the updated boot.cmd from 5.25/5.26/5.27 gc2035/vfe_v4l2 stop to work (croaking about CSI not able to detect a target chip).
That's all folks!
Now should someone versed in u-boot parameters arcanes invest some time to see what's the real difference which impacts gc2035/vfe_v4l2, and eventually corrects/documents it to everybody potentially banging their head about this in the future ?