[JasonB]

It's all to do with EXIF orientation tags. A lot of modern devices record orientation information as part of the image file. The viewing software then reads that information and rotates the image when you view it. If you tell the viewing software to rotate the image, it's often just the orientation information that's altered, the image itself may still be the wrong way round. I'm fairly sure that's how Photos on Windows 10 works.

That's all okay if you only ever use software that reads that information, but you'll get problems on anything that doesn't.

Here's a piece of software which is supposed to actually rotate images so that they work on everything, and it can be applied to whole folders, and it will automatically detect whether a photo needs to be rotated or not.

https://savolai.net/software/jpeg-autorotate/

Test it on a few images first, before you go mad with it. I've never used it so I can't vouch for it, but it might be worth a try.