Hi everyone

I have a very simple configuration:

• One FTTH router with dual channel 2.4 & 5 Ghz

• Mikrotik 951G-2HnD connected via Ethernet and serving Wireless as AP Bridge in 2.4Ghz

As I move through my home, mobile devices do not change from one AP to the other, and they even stay with one AP until they don't have connection, so they go to 4G and then they connect to the nearest AP.
When I had a Wireless Extender in the past, wireless roaming was done correctly, and as the signal from one AP got weaker, devices connected to the one with stronger signal, but now with Mikrotik there are times it keeps on connecting to the FTTH router even when I'm closer to the Mikrotik.

Both router and Mikrotik have the same SSID, same security (WPA2 with aes and tkip) and different channels

Any hint on why there's no roaming between APs?

In such a configuration, roaming is entirely up to the client. When it does not look around on other channels and/or does not decide to hop to a stronger signal, no roaming is taking place.
With your extender both APs were probably on the same channel and the client may behave differently.

So if I move the Mikrotik to the same channel, it may change behavior?

I don't fully understand what causes the client to behave in one way or the other...

Nobody does. You will have to test it and see what happens.