Thanks for your reply, and yes, I think you're right. I didn't think about it before as being the cause of the problem.
I included two other screenshots, this time in Taber, Alberta, Canada, and Haigler, Nebraska (US).
In Abu Simbel, which is at latitude 21 N, the circles are stretched in the east-west direction.
In Haigler, latitude 39 N, they are pretty much round.
In Taber, 49 N, they are stretched in the north-south direction instead.
Which reminded me immediately of those highly unrealistic clouds stretched north-south you see when flying in the Arctic in FSX. So that's it, this is caused by the geometrical limitations in the way FSX renders the spherical planet. I have no clue if this problem has been solved in the new MSFS, but I hope so.
Thanks for you time, have a great day.
Edit : it looks like FSX was calibrated to give perfect texture rendering around the latitude of the central continental US.
Luc