Hi there! Only just saw this. I had a similar thing happen with DD.
At 8 weeks the MCHN said to keep a real close eye on it and do everything I was already doing. I decided to take her to an osteopath. From her first appointment, I noticed a change. DD no longer preferred one side of her head to the other. She has now had 4 appt with the osteo over 3 months and the flat spot has nearly gone. I credit it to the osteo.

I've also got a Bumbo that DD loves, and she enjoys tummy time for short periods. Once she starts sitting on her own, I think it will completely correct itself, but If I hadn't have seen the osteo, I'm not so sure it would have worked out this well.