Hello Roberto
After you configured the R3 router with Area 1 as a stub, if you were to issue the show ip ospf neighbor command, you would find that R2 and R3 were no longer neighbors. This is the reason that you saw no OSPF routes in R3 after that change.
Once you made Area 1 on R2 a stub as well, the adjacency was reestablished, and that’s why you saw the networks reappear as expected in R3.
OSPF will form neighbor adjacencies with other OSPF routers only if the area type matches. This is one of the parameters found within the hello packets that must be the same before an adjacency can be formed.
More information about this can be found here:
I hope this has been helpful!
Laz