I’ve had this happen to me a couple times but I keep forgetting to report it. It seems if I enter a horse in an upcoming race then scratch, the horse then disappears from some of the race entry dropdown menus when I try to enter them elsewhere.
Example:
Just now, I entered Downfall in race #25 on 04-18 Aqueduct, then realized race #21 would be a better fit (same date and track). Downfall is currently at Aqueduct. After I scratched from race #25, I couldn’t enter him in race #21. Both are open races, 4yo+. #21 doesn’t appear on his Upcoming Races page, either.