My pet hypothesis: people who are into one of the many athletic pastimes, particularly those with a competitive component, will all do exactly one thing when faced with an appointment that comes with an attached "take a rest from exercising afterwards": they will schedule a particularly intensive workout the day before so that those rest days "don't go to waste". (source: I'm one of those and very few in my social circle don't match that pattern)

But the risk association might very well be not so much the exercise happening while under immune stress but the processes that happen during recovery doing their thing while under immune stress. Usually immune stress means infection and we don't get to schedule our infections, so this distinction (exercise time vs recovery time) will be very much unexplored.

Young males tend to be more into performance-oriented training (competitive, or focused on metrics, or both) than other population groups, and with an actual infection it might even be that the relevant immune stress does not really start before more direct symptoms suggest taking a break. Could be as simple as shifting the "no sports" instruction that comes with the jab a few days backwards.