Interestingly, I tried changing another option in the menu, although can't fully remember what it was, but it was something to with selecting between Station name and Type. I think it was defaulting to Name so I changed it to Type. Although it was a short journey when I did this, initially I am hopeful it may have stopped it switching back to FM from DAB. But time will tell.
Just to clarify, this happens when moving up/down through the Favourites via the steering wheel selector.
But to answer your questions: when I said DAB is better, I agree it drops out, and does so a lot in several 'cold' spots where I live (Weybridge, Surrey), but when not in those areas the sound quality is generally better in DAB mode in terms of lack of interference and frequency rages. That difference seems more acute in the Stinger, funnily enough. I have had numerous other cars where, if anything, it was less noticeable or even the other way around. That said, I think DAB has improved a lot over the years. I remember some years ago some stations had terrible sound quality in DAB, like Capital, but have improved. I could liken it to having been ripped as an Mp3 file in a very low bit rate - really thin sounding. With my misophonia (extreme sensitivity to noise) it was really uncomfortable. I guess they have improved the bitrate of most DAB stations. I think far more are at least 128mbps now but that didn't used to be the case.
I agree that a really good FM signal will sound 'warmer' than a poor DAB one though. In the same way and audiphile buff would say a vinyl LP will sound warmer/better than an mp3 recording.
Back to the car though, and, Radio 1 seems to have very poor FM signal in the Stinger around here which surprised me as BBC stations are usually the strongest. And you asked about Regional - I had tried both on and off but made no difference.
Hopefully it's ok now otherwise I will mention it at its first service next week.
I have a list of minor issues already. Now have a tyre going flat but can't see any punctures anywhere so suspect a faulty valve. if so they can do it under warranty. Otherwise I would just go to a tyre fitter.