Many reports said that Diego Simeone did not like the player, Felix statement that he wanted to cross to Barcelona added to the heat of the situation. The price set by the ATM for the player is the reason for the complexity of the Portuguese striker's move. On the other hand, this is very reasonable, because Felix was bought at a high price from Benfica in 2019.
If you look at the price on Transfermarkt and compare it with its current performance on the field, it's too expensive. Agree with you 40 Million Euros is perfect for Felix now.
40 million is under the market value because currently Joao Felix's value is 50 million, I'm not sure Atletico agree with that price especially he is a young player of 23 years, regardless of decreased performance it doesn't describe his future I mean he still has time to get it back. After all, moving to another club is the best way to prove that letting go was a mistake.