I had a kind of similar experience. I used to do sprint work and speed training once a week throughout the off season and race season. My sprint would usually plateau at around 1200 watts and I’d gradually feel like it was getting worse. It wasn’t until the first covid lockdown last year where i only did zone 2 for 8 weeks straight and then hit 1400 watts on my first sprint in two months that i realized the dedicated sprint training didnt work for me. Since then I basically only do aerobic training with form sprints just to keep the technique good and then save my max sprints for races or group rides. I still consistently hit 1400 watts which was unthinkable before.