Not sure if this helps ...
As soon as you increase the MTU on the ax0 interface by four bytes, does your throughput stabilize as expected?Logging error messages may significantly affect performance, especially if they are on a per-packet basis.Furthermore, could you try enabling hardware VLAN tagging in interfaces -> settings?. Though this logically shouldn't be necessary given the expected systems' capability of dealing with both scenarios, I think an incomplete assertion is being done in the driver code.
I couldn't find HW VLAN tagging in interfaces -> settings (or would this be included in VLAN HW Filtering?)
Sorry for the novice question, but how could I best make the HW VLAN tagging persistent? Happy to check what's happening with logging reduced if you could please give me a hint on how to accomplish that.
I also tried to get rid of the logging but wouldn't be able to see where (axgbe_debug_level is 0 and Interfaces->Settings->Log level is standard).