I don't think its a bug, maybe general mistake.
Players make new highlights in Cup games and thats show in the Best Performances of the league.. I don't think that have to be like that..
Maybe it include also SC records, I don't know.. but something wrong with that system..
Players from teams history which didn't played one minute in the league enter to the Best Performances just for past record which didn't set in this league..
for example in our league:
"Rebounds: 31 by Massimo Zanellato"
this is a record he made just in the last Cup game..
Hope for answers ..
Thanks.