Has he really been out of form though? Or has his positioning changed based on Ancelotti's requirements of him?
In the two Man City games he was used as the primary defensive midfielder, with Kroos being the main outlet for long passes for Vini/Rodrygo. Even Valverde was shunted to a defensive winger position whose main duty was to track back. This was especially apparent in the second leg, after Tchouameni was suspended so they had to play Nacho and Bellingham and Camavinga ran their absolute guts off protecting their back line.
Look at the heat maps of Bellingham in La Liga: even the recent ones against Barca when he was way up the pitch.
https://www.sofascore.com/player/jude-bellingham/991011
Then look at the heat maps of him playing vs Man City and Bayern.
The majority of his position in both city games were in his own half, and in the Bayern game it was the center circle, the right defensive side of his half and the offensive left side of Bayern.
Compare that to his La Liga games where he barely touches the ball in his own defensive third.
This is happening because he is a #8/10 who just so happens to have a gigantic engine, great physical abilities, positioning and tackling that Ancelotti decides to use him as a defensive midfielder in the big games when he has key defenders missing. Of course his offensive game is going to suffer as a result.
What do you think will happen if Musiala, Foden or Odegaard is asked to play that role? I can guarantee that they would be just as ineffective offensively but far far worse defensively too.
People are basically complaining that a guy who has been playing as a shadow striker/false 9 for the majority of the season is no longer as effective at the top end of the pitch when he is being played as a #5.
The expectations people are putting on him is absolutely ridiculous.