It certainly appears that way, given the statement that he released on Sunday. He is Prefect for the Congregation for the Doctrine of the Faith. He said that a "fraternal correction" regarding Amoris Laetitia is "not possible" because AL poses "no danger to the faith". Edward Pentin, in the National Catholic Register, points out that the CDF lodged a large number of corrections for AL; not one of them was accepted.
Now what else happened at the CDW just a few days prior to that? Recall that on December 26, the pope ordered Cardinal Muller to dismiss three of the priests who were working with the CDW. When the Cardinal succeeded in getting an audience so that he could discuss the matter with the pope, the pope tersely told the cardinal, "I am the pope, I do not need to give reasons for any of my decisions. I have decided that they have to leave, and they have to leave". He then walked out of the room, ending the meeting right then and there. So much for all this "collegiality". It seems reasonable to guess that Cardinal Muller felt the pope's displeasure. One might think of the fear that a mafia underling would feel if his don behaved like that.
Like Mundabor, I strongly suspect that Muller is in "cya" mode: that is, "cover your @ss". While I can understand Muller's "about-face", I can in no way condone it. I had hopes that he was a bright spot in a very dark place, but he has cowed; no John Fisher is he. Maybe he will reconsider; I pray he does. But if he does not, let the four dubia cardinals press forward and do what needs to be done to bring some light and truth to the higher echelons of the Church. Maybe Cardinal Napier can join them with his own dubium.