Yeah I hate this too - one of my clients regularly supplies images as HEIC files and they are always crappy to work with. Plus the images just seem much lower quality, which is a problem when they are being used for professional purposes.
Maybe, although it's multiple people, so seems like it wouldn't be a camera fault.
Possibly it's how they are getting to me - I don't get them directly, they come to via someone else, and I think the someone else may be getting them via WhatsApp (which I don't use).
So in fact it's maybe the compression from WA, rather than an issue with HEIC? I should have thought of that before!
I’m sure you don’t need telling, but just in case, I would check the meta data to see when the last time the file was edited. If it matches when they sent it to you, that’s the cause.