Hi @grootwitbaas Welcome to the community!
Be sure to checkout its guidelines 
https://community.icinga.com/guidelines
Have you tried to restart the master2 like this:
systemctl stop icinga2 && rm -rf /var/lib/icinga2/api/zones-stage/* && rm -rf /var/lib/icinga2/api/zones/* && systemctl start icinga2
You dont seem to have any problems in your syncing, but It will force a resync between the 2 masters from master1 -> master2, It might not solve your problem but its worth a try I found it here originally:
https://community.icinga.com/t/last-zone-sync-stage-validation-failed/