Hello Azm
The process of learning MAC addresses takes place all the time. When the port is in the forwarding state, it still learns MAC addresses. MAC address learning is not restricted only to the learning state.
The purpose of the learning state in the original STP is to prevent network loops during the transitional period when the spanning tree topology is still being established. By not forwarding frames immediately, the switch can avoid creating loops that could occur if frames were forwarded without a clear understanding of the network topology. The duration of the learning state is typically set to ensure that the switch has enough time to populate the MAC address table before starting to forward frames.
Where STP used a timeout to ensure this loop-free topology, RSTP+ uses a request/response mechanism which is faster. The learning stage is almost instantaneous because the switch sends an RSTP proposal message immediately and receives one right away. If there are still MAC addresses to be learned they will be learned in due course whenever a new frame arrives on a port of that particular switch.
I hope this has been helpful!
Laz