Because it has been a while, and during this winter event where the value of coins is much higher (with the fee it has become quite difficult to get as much profit it seems), this bug is kinda game-breaking :
if you have opened the association widget and refreshed it just after the bonuses have been collected (therefore all the videos are available), you can then watch 1 video of each of your teammates that doesn't exist anymore (because someone else watched it after you refreshed, but before you clicked), and still gain the bonus from the second video (which is the best way to gain winter coins atm).
Basically, instead of 1 person gaining 25 bonuses, you can have as many as 25 people gaining those 25 bonuses each.
It's annoying for 2 reasons :
1/ in regular times, you might lose time watching videos without giving your teammates any profit (because someone else already did)
2/ you can get much more bonuses than should be possible without this bug
Do you mean you can still watch videos that were already watched or you can't?
If you can watch all of them still that is working as intended and not a bug.