Ok. Here’s the story. If my assumption is correct.
When you apply import-policy on one MX router to increase local-preference for default-route to 200, it will advertise this default route to other iBGP peer (second MX).
Now Second MX has 2 router one with default local preference 100 received from eBGP peer and another from iBGP with local preference of 200.
Due to BGP path-selection process, second MX prefer the default route received from the iBGP peer instead of eBGP peer and install it in it’s routing-table.
Since BGP only advertise the Best route, another mx will not advertise the default-route received from eBGP peer to iBGP because that is not the best route. This is the reason you only see one route on first MX is is correct behavior.
HTH
When you apply import-policy on one MX router to increase local-preference for default-route to 200, it will advertise this default route to other iBGP peer (second MX).
Now Second MX has 2 router one with default local preference 100 received from eBGP peer and another from iBGP with local preference of 200.
Due to BGP path-selection process, second MX prefer the default route received from the iBGP peer instead of eBGP peer and install it in it’s routing-table.
Since BGP only advertise the Best route, another mx will not advertise the default-route received from eBGP peer to iBGP because that is not the best route. This is the reason you only see one route on first MX is is correct behavior.
HTH