On P10 lite in renders mobile site perfectly in portrait and landscape mode.
On Pixel 4a it renders perfectly in both modes, but interestingly when rotated to portrait mode in shows the desktop site, but it all works fine.
Watching how the site is constructed. The pictures and text are downloaded a fraction before the CSS so the layout for a second is not right. If this user has a poor signal or a phone clogged with rubbish then it may get stuck part loaded? Just a thought and I don't think you have a problem at all.