topology : R1 - R2 - R3
R1-R2 : ospf neighbors
R2-R3 : ebgp neighbors
routes when redistributed from BGP to OSPF on R2 are being tagged, but the default-route is not getting tagged.....
The default route is not redistributed into OSPF, it is only originated into it. Even though you may be performing redistribution from BGP into OSPF, a default route cannot be redistributed into OSPF (it is a deliberate restriction in Cisco IOS). This is the reason that you do not see a tag in the default route - because it is actually not retaken from the BGP.
If you want to tag the default route originated into OSPF you have to use a route-map in the default-information originate command in your OSPF configuration to tag this route, as follows:
route-map RM-TagDefault permit 10
set tag 123
router ospf 1
default-information originate route-map RM-TagDefault