I’m having the same issue. My Sim has maxed out the fitness skill, and when I have him work out on the bike, he only pedals for about 4 seconds before stopping and idling for a long time. After that, he briefly pedals again, then stops once more, repeating this cycle endlessly. I even tried sending Bella Goth to the gym (she’s level 10 in fitness as well), and the exact same thing happened to her. Other Sims that I’m not controlling pedal normally without stopping. They probably don’t have the fitness skill maxed out.
I tested the game in a fresh The Sims 4 folder, and the issue still persisted. I tried it with Bella Goth and even recorded a video showing exactly what happens.
I also tested with the Pancakes family, and after a few interruptions in the pedaling animation, Eliza eventually began pedaling continuously and behaved normally. However, this doesn’t always occur, most of the time, Sims remain stuck in that constant stop-and-start cycling loop.
VIDEO LINK