OSPF advertising loopback network
OSPF will always advertise the networks on loopback interfaces as /32 networks, even if you have configured them with a different subnet. This is how OSPF operates, and it is even defined within the RFC. If you want to change this behavior, you can use the ip ospf network point-to-point
command under the loopback interface configuration mode. This will cause OSPF to advertise the configured subnet mask of the loopback interface.
Links:
https://www.ietf.org/rfc/rfc2328.txt
https://forum.networklessons.com/t/mpls-layer-3-vpn-pe-ce-ospf/1291/60?u=lagapides