I have a Toro model 21386 and the belt came off two weeks ago.  Putting the belt back on was a learning curve as there were no YT videos for this model or anything close.  I tightened the tension cable after marking the original position with a Sharpie.  I mowed yesterday and when I was almost finished, the belt slipped off again.  I suspect that the belt is getting stretched and loosening.  I have ordered a replacement OEM belt except that this replacement is made with Kevlar. Hopefully, this newer belt will last longer. And I hope to reduce the installation time to less than an hour.  I believe that this Toro is a 2020 model, I purchased it new in March 2023 from a closeout store for $350 with no warranty.  My 2020 Honda HRN216VKAA runs well with no belt issues (the belts are Kevlar), and the belt is more exposed than the Toro.