We see all routes for a split second and then we then receive these errors on the 7.5 router: received wrong LS Ack for inter-area-prefix 10.10.0.0 10.10.9.14 0x80000006 expected 0x80000007.
This then brings DOWN the gre tunnel entirely, which is very interesting as without OSPF the tunnel is stable.
After this the gre tunnel comes back online and we start the loop again.
You probably already got this figured out, but it sounds like OSPF is installing a route for your external IPs to go across the tunnel. The moment the routes get installed, your routers start sending your tunnel management traffic across the tunnel. You need to filter out the external IPs of the tunnel from OSPF using a Route Filters. You can see my related post about this over here: viewtopic.php?p=961905