Most likely that the Hyoer OS released for the older MI models which have G dialler do not have the new MI dialler.
Quantum_Buddy
Explorer
I have Redmi 12 5g it also has custom dialer.
.....
So, I think afterwards 2023 they shifted back.
Any model by Xiaomi after 2023 will have their own dialer. But do buy after viewing a review or two.