Since September 2024, Prusa has released new O-rings for the Prusa XL belt idlers.
So far there is no official reason for this from Prusa that I know of. After some research I was able to find Reddit and in the Prusa Forum find out more.
Prusa has set up Printables published newly adapted CoreXY parts. I compared the parts with the old parts and found that the new ones now have 1mm more clearance on the axle side.
The new O-rings are therefore approximately thicker than the old ones. The new O-rings have a thickness of 1.2 mm.
Apparently there were problems with the old O-rings and the insufficient clearance, which led to the O-rings wearing out quickly and causing irregularities in the print image and unwanted noises.
I myself and others have been printing with the old O-rings for 1.5 years and have not been able to detect any faults.
However, we recommend reprinting the modified parts during the next extensive printer maintenance and integrating them together with the new O-rings. At the same time, you can also maintain or, if necessary, replace the pulleys.
You may also have noticed that there are two idler pulleys in the X carriage, where the belt is guided with the toothed side over smooth idler pulleys. Why Prusa does not use toothed pulleys for this is a mystery to many. This is because the belt can become deformed. It is therefore advisable to use toothed idler pulleys. All parts and information can be found here: https://www.printables.com/model/1135912-prusa-xl-modified-corexy-parts-for-new-o-rings-wit
English 
Deutsch