I’ve also just noticed that it cannot download more than one file at a time.
My mistake. What was actually happening is every time you download or save something, the file requester reverts back to the xdg-portal temporary directory in /run/user, and it refuses to save there, because that’s where the temporary file goes… so it does that every single time you download something, and I forgot to change it. Rather than display an error message, this simply causes the downloads to silently fail.