Is it true that two CRS317-1G-16S+RM devices are involved in this test?
Why not testing on a single device first?
If really two are involved, then they better should be in their own LAN (ie. IP should be something like 192.168.88.1/24 and the other should be 192.168.89.1/24). At least for the testing.
It seems the traffic goes over your 1G gateway address
Hmm. maybe not as it's 1.34Gbps.
I think it's a routing problem.
The outputs of these would be needed for further analysis:
/ip route print
/ip address print
And the IP+mask of all involved devices / interfaces.
Another observation: it seems you used iperf for testing, but does it not have also a SUM line under the ID heading?