Update: I’ve narrowed it down a bit more. I installed bluestacks on my PC, and voila:

Instant direct play, no arguments. No bandwidth limitations. The same IP. The same network connection.
I also took the liberty of trying to play something on a completely different Windows PC (vLAN 10) and… got the same results on Plex for Windows. I then went to Plex Web, same results. I switched profiles to the family profile, and VOILA! Full streaming capabilities. I then switched profiles on Plex for Windows, played the same file… limited. To make sure it wasn’t a fluke, I tried again on Plex Web. Success. I then went back to my personal PC, switched to the family profile, fail…
So, to recap…
- I am bandwidth limited on my PC, despite which vLAN I’m on, and despite which profile or client I’m on.
- However, if I emulate the plex android app via bluestacks, I can direct play with no limitations.
- On another completely separate PC, I am bandwidth limited on all profiles on Plex for Windows.
- However on the family profile on Plex Web, I am not limited. This PC is on vLAN 10 as well. Ethernet.
- Despite that, on my PC, the family profile doesn’t grant magical non-limits.
Plex logs of the whole ordeal: (File removed)