Okay, took longer than I'd hoped to post this.
I set the difficulty to very easy and had basic mission armaments (one subach, one morning star, 2 banks of tempests) in the default Perseus. The morning star allows me to win against Delta wing for the Oberon defense (by pushing them, they stop firing and move off) in case getting the secondary objective might trigger something...and it happened yet again.
I've noticed this happening in any campaign that I play. I even tried doing the same mission in FSBlue. Also the "spinning ships" issue occurs pretty much in any mission of any campaign at random. Usually doesn't impact gameplay except it being funny watching fighters pull off insane G's with these spins.
Here's the log:
https://fsnebula.org/log/62cc4d54d58598f151f3408bAnd it seems Delta wing does get the order to leave...they just don't.
Lines 3987 to 3992:
Entered mission_do_departure() for Delta 1
Acquired departure path
Entered mission_do_departure() for Delta 2
Acquired departure path
Entered mission_do_departure() for Delta 3
Acquired departure path
EDIT: I need to attempt the mission in Retail just to see if it's also doing it there.
EDIT 2: Just thinking out loud here, but could this be physics-related? Again I'll try in Retail later. I need to head to work, but I'll make a Retail run when I get back.