Thank you for your reply.
3.5.1, simple, WIP: No, Thumb: No, Crop: No, Resize 400x400 90% quality.
I have just checked the EXIF data of one that was being rotated, it is being caused by the orientation tag. This is a self-service form where members can upload their own photos to accompany news items, so we have no control over pre-preparing the uploaded files or the EXIF data they might contain. We really need to assume their photos are the right way round and not to rotate any of them. A setting option in the plugin would be very useful.
In the meanwhile, do you happen to know which file in the plugin contains the rotation code so I can comment it out, please?