hmmm… i’m not sure what to make of this. when i try to upload a .heic file via drag+drop, i don’t even get as far as you did:
there’s no observation “card” created in my upload page. it just fails with an error message.
…
it seems like the Mac (not iNat) must be doing some sort of conversion on the fly when you try to upload your photos. but there might be some sort of weird timing thing / race condition where that conversion isn’t complete before iNat attempts to use the file.
there’s another thread where @sullivanrabbit proposed this workflow for exporting photos (https://forum.inaturalist.org/t/random-iphone-photos-wont-upload-to-inaturalist-from-macbook/17857/3), which seemed to solve the problems that the original poster on that thread was having. the problem described there sounds similar to what you’re describing here. so you could try that workflow and see if it solves your problems, too.