Default route missing after switching MASTER/BACKUP status

[lodur]

Hi!

We are using two machines in High Availability Sync mode (pfsync and XMLRPC Sync) with CARP VIPs on LAN and WAN Interface. Both WAN and LAN Interfaces are configured as a LAGG with LACP consisting of two NICs.
Today I needed to replace the switch on which the WAN Interfaces are connected to. I started with the current Backup machine, switching to CARP Maintenance Mode and connected the corresponding NICs to the new switch. After the links and interfaces were up again, I left maintenance mode and did the same for the Master machine. The Backup machine switched to Master without any issues, all the traffic was routed correctly, everything looked perfectly fine.
After the migration to the new swtich was done, I left maintenance mode on the regular Master machine. It switched to master, the Backup machine switched to backup again, so far so good.
Then I noticed that no traffic was routed through the Master machine and after some invesitgation I found out that the default route was missing in the routing table.

What went wrong? Did I make some mistake?

Thanks in advance!

Best regards,
Michael