So I can manually exercise, but only up to a certain letter in the alphabet. I just set 6 horses to exercise lightly: El Trujaman, Magic Maker, Major Type, Personal Space, Seshat, and Speeding Ticket. Only El Trujaman was exercised. The others did not appear in any way, even as an error message or only on their personal records. I tried exercising them one at a time and it didn’t work either. So somewhere between ‘E’ and ‘M’ things get weird.
This is a known issue for larger stables, basically there’s only so much workout data you can submit at once so it throws out anything too large. I still need to figure a decent workaround.