Bellingham was the top scorer of Madrid's squad last season. Bellingham is not in good form this season. He has only scored 4 goals and 3 assists in 11 matches. In addition, Vini is now facing injury again. Mbappe is not getting the expected good performance. Even Madrid has lost points in several matches due to Mbappe's bad performance. It is not possible to say for sure whether Madrid will be able to play well in the upcoming matches of the Champions League.
I think Bellingham decline has to be caused by Mbappe coming to the club, he doesn't have time on the ball anymore as he gets to create chances for Mbappe and others to score. We are the one making it look like Bellingham isn't playing well as he's no longer scoring. We forget that Bellingham is a midfielder and not a striker. The goal scoring are just extra benefits he used to bring to the club but his primary goals is to create chances and balance the midfield. I was disappointed with Mbappe in the last Real Madrid game for missing another penalty. It looks like he'll allowing the pressure to get to him at Real Madrid. He needs to relax himself and step up more often.