I confirm the same problem (on iOS platform).
Download works fine if selecting a single item (1 movie at a time).
However when trying to download a full season of a TV serie or a full music album (click download on the season or album itself), the first few items are downloaded correctly and then it slows down and retries continuously.
It’s not random, it happens every time, so it should be easy to reproduce by Plex engineers.
This needs to be resolved as offline watching is one of the main features used by plex users.
@Plex, at least please acknowledge and confirm this is being investigated.
Thanks.