I think there was only a small problem between Neymar and Mbappe last season because Mbappe accused Neymar of his ego on the pitch. After that incident I think they are in good shape, indeed on the one hand if a team has internal problems between players I think it will cause a big problem in the team's journey to win the title because they are not balanced in concentration on the pitch.
Neymar has been closely linked with Chelsea in the past few weeks but until now it seems the rumors have not come true and I think he prefers to stay with PSG this season.
Every great player clearly has a little ego to be better in the team and also on the field, especially if Neymar and Mbappe are great players who have become stars at PSG.
So it's natural that there is a little ego and it becomes a problem, but PSG club officials and coaches must have resolved this in a healthy manner.
As for this rumor I haven't read it and also Chelsea have not shown much interest in Neymar. Although there has been news about Neymar being linked with Chelsea now.