The headline in an article by by André Schulz at Chessbase says it all:
Kasimdzhanov after ending collaboration with Caruana: “It has a lot to do with the pandemic”
9/2/2021 – Fabiano Caruana and his second Rustam Kasimdzhanov formed a very successful team. However, their best year together (2018) ended in disappointment after many successes. The last Candidates Tournament did not go as desired either. “The pandemic year did not do our relationship any good”, says Rustam Kasimdzhanov. Now Fabiano Caruana and Rustam Kasimdzhanov are going their separate ways. (https://en.chessbase.com/post/rustam-kasimdzhanov-interview-2021-no-longer-caruana-second)
When asked, “And how are things going between you two now?” the Katzenjammer Kid
replied, “It’s strange. It’s like a divorce.”
Dedicated to George Ziberna and Gail Childs, may she R.I.P.