When using Mitch on FireOS, downloading never starts. Not sure why, the download provider seems similar to AOSP though they must have messed with it somehow.
Really strange since Mitch doesn't even use the Android download manager. I use OkHttp3 and directly handle downloading within the app (this gives me more control over where the files get downloaded). You'd think this would be OS-agnostic.
Can't really debug this but maybe you can look into OkHttp3's GitHub issues?