RESOLVED: it was caused by the app being flatpak. use Flatseal!
I have a network drive mounted to my computer (PopOS). I have read & write access to everything in the drive and can drag and drop files between two Nautilus windows, but if I try to drag a file from the network drive into an email for example, I get a popup about file permissions. Is it because flatpak apps don’t inherit my user permissions?
I suppose it might be doable; something could throw a pop-up that says “Flatpak app XYZ doesn’t have access to this folder. Do you want to enable access for this folder only, or all files?”