A dirty filter causes banding in a blue sky? That's the first I've heard of that...Filter, dirty filter, or dirty lens are potentials.
Yes, I shoot RAW. I turned off ES and took 10 shots this morning at 8mm in similar skies and they were fine when all shots at 8mm the other day were banded, but other lengths were fine. My guess is that it was the ES at 8mm although I have no understanding of why this would be but I'd need another full day out at 8mm with ES off to confirm.A dirty filter causes banding in a blue sky? That's the first I've heard of that...
@Bidkev , While the electronic shutter may be contributing to the banding, I'd be surprised if it were because the E-shutter on the E-M5 II shoots 12-bit RAW (I'm assuming you shot this in RAW and then edited, correct?), which is the same as the mechanical shutter.
As for why it would be worse at 8mm vs 16mm, it may be that due to the wider FoV, you're capturing more of the sky and thus the camera has to deal with more gradations of color in the sky (a narrower FoV means less gradation in the frame, so less potential for color banding). As far as why it happens, I can't help you there unfortunately..