Sorry for the multiple replies, but I think I've found an anomaly.
When uploading the same HEIF file via the Windows app, it works just fine on 3.1.16.0. I can see the thumbnail, and it's uploaded as a HEIF photo. It also shows up properly on my other devics.
When uploading via the iOS app, it gets uploaded as a JPG image, not HEIF, so that works fine across all devices too.
When I upload it through the Android file picker though, it uploads it in HEIF format and there's no thumbnail on any of my devices.
I just sent a follow-up email with a screen recording for clarity.