@Brd709 Can you do these things to narrow it down?
1. Create a new Sims 4 folder, so that it's clean since you last had the bug (drag the folder to your desktop then start the game). Don't put back any saves or mods/CC to start.
2. Create a "New" game. Go to the gym and check how time behaves. Now start a stayover and do the same thing.
3. Now exit and copy your old save over. Start again, opening your old save. DON'T start a stayover; just go to the gym. Then DO start a stayover and go to the gym.
In any of those 4 scenarios, does the bug recur?