I have a problem with connection speeds when connection v12 to v11.2.1

When connecting v11.2.1 (on the same device) to the same v11.2.1 i get a steady 115-120Mbps between the two.

I tried disabling the QoS on one, both, enabled on both.

Whether QoS is enabled on the remote v11.2.1 makes no difference (the remote location has 150/150 fios).

When QoS is enabled on the local v11.2.1, the speed fluctuates from around 35-40Mbps to around 80-90Mbps (averaging around 65-70Mbps).
When QoS is enabled on the local v12, the speed fluctuates between 18Mbps and 36Mbps.
When QoS is disabled on the local v12, the speed fluctuates between 26Mbps and 42Mpbs.

Is there a known compatibility issue between v12 and earlier releases?

I'm at a loss here.
Should i use the v11.2.1 on both and not upgrade (when the upgrade is available).
Or should i upgrade both and hope for the best.

Unfortunately don't have a second v12 to test the speeds between them to see if it works better.

Thank you in advance for any input.