I'm not mad at you. Sure I'm not manufacturer, and I don't spend as much time as you on Amiga hardware.
It's just that I would have sent my Amiga motherboards for nothing.
What I have discovered is interesting enough to put us in the right way for using those screenmodes.
No I didn't solve by changing a WB setting: In my 1st video not only the backdrop has glitches but also the windows with icons.
Well, I will do some cross tests with my 2 boards and will do a full report of what I did from the begining.
PS: I haven't seen any video with the same problem I had. (horizontal glitches)