My guess is that your image file may have orientation data in it (in EXIF) that lemmy strips out when it saves it. If this is the case then IMO it may be a lemmy bug or a bug in the client you're using.
You may be able to avoid it by stripping the EXIF data and ensuring the image is properly oriented the way you want. Not sure what's available for iOS to do this for you.
I'm curious are you using the website or a mobile app?