In the video you've linked to, the spheres are seen on both the leading and trailing edges of the propeller:
I expect they are intentional - there are a number of ways to attach a propeller without having to disturb that surface, or using flush caps.
Cavitation is caused by a drop in pressure. This would be seen on the trailing edges of the propeller, and is worse on the outer edge of the propeller which is moving faster through water than the inner area. I doubt these spheres have any effect on cavitation at all.
It is either a sensor system, or noise reduction. If sensors, it's possible that these could be used as a rearward doppler array, even on a moving propeller, though this seems unlikely.
More likely, though, is not just a passive noise reduction system, but an active noise reduction system. These may be transducers that emit sound intended to cancel out specific, otherwise unavoidable noise the submarine generates during normal operation. Active noise suppression systems have been in use in silent submarines for decades now, but are, of course, kept secret.
Another possibility is that these are caps that are covering something else, and are removed (manually or merely fall off by using the propeller in water) prior to active duty. They may be hiding parts of the propeller design not intended to be public. Given that they are made of a distinctly different material or coated differently in some photographs, this might be the case.
Lastly, they might be there simply to throw off competitors and are red herrings.
Unfortunately the secretive nature of stealth submarine technology means we may not be able to definitively prove any of these hypothesis.