RAX200 and WAN Link Aggregation + Multi-Gig Port

It appears that Comcast updated the CM1150V to firmware 4.02.02 which now provides an official way to turn on link aggregation. I enabled it on my RAX200 (updated to .102 firmware - which at least seems to have IPv6 working again even if there are still too many problems to justify the price of the router) and things worked great and I was seeing 1Gbps+ (Comcast over provisions a little on their 1Gbps connection) through the router selftest.

But then I checked on the other end of my backbone which uses the multi-gig port to connect to a Netgear GS110EMX (another long story here about how the RAX200 static lag will not work with GS112Ev2 static LAG - that case has been open for 8 months now) and was seeing only 500 Mbps.

Switch off WAN LAG and I go back up to 1Gbps on the other side of the switch. It appears I either get WAN link aggregation or decent performance on the LAN side with the MultiGig connection. Note the regular 1 Gbps connection tests fine as well with WAN LAG enabled.

Any ideas if this is a supported configuration? Already checked the docs and they are useless as usual.