From what I was reading immich should preserve your original media. I saw this bug report[0] but it’s not clear if it’s an app upload bug, iOS bug or maybe depends on the device??
I don’t use immich (yet) but this is the kind of stuff I worry about. I’m planning to use it in read only mode though and sync my photos using PhotoSync rather than rely on the app.
It has botched slow motion uploading. It uploads an export at 30fps instead of maintaining 120/240fps.