Hmm, I can replicate your setup very closely here but can't replicate the problem. Yet.
This is an X520 NIC connected as a LAGG to two 10G ports on ICX-6450 using Brocade DAC cables:
[22.01-RELEASE][root@7100.stevew.lan]/root: netstat -i
Name Mtu Network Address Ipkts Ierrs Idrop Opkts Oerrs Coll
ix0 1500 <Link#1> 8c:dc:d4:a8:15:e8 259154487 0 0 259272608 0 0
ix1 1500 <Link#2> 8c:dc:d4:a8:15:e8 240144841 0 0 240127722 0 0
This:
dev.ix.0.%pnpinfo: vendor=0x8086 device=0x10fb subvendor=0x103c subdevice=0x17d3 class=0x020000
dev.ix.0.%location: slot=0 function=0 dbsf=pci0:2:0:0
dev.ix.0.%driver: ix
dev.ix.0.%desc: Intel(R) X520 82599ES (SFI/SFP+)
[22.01-RELEASE][root@7100.stevew.lan]/root: ifconfig -v ix0
ix0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=e138bb<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,VLAN_HWCSUM,WOL_UCAST,WOL_MCAST,WOL_MAGIC,VLAN_HWFILTER,RXCSUM_IPV6,TXCSUM_IPV6>
ether 8c:dc:d4:a8:15:e8
media: Ethernet autoselect (10Gbase-Twinax <full-duplex,rxpause,txpause>)
status: active
nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
plugged: SFP/SFP+/SFP28 1X Copper Active (Copper pigtail)
vendor: BROCADE PN: 58-1000026-01 SN: CAX112240004092 DATE: 2012-06-16
[22.01-RELEASE][root@7100.stevew.lan]/root: ifconfig -v ix1
ix1: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
options=e138bb<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,JUMBO_MTU,VLAN_HWCSUM,WOL_UCAST,WOL_MCAST,WOL_MAGIC,VLAN_HWFILTER,RXCSUM_IPV6,TXCSUM_IPV6>
ether 8c:dc:d4:a8:15:e8
hwaddr 8c:dc:d4:a8:15:e9
media: Ethernet autoselect (10Gbase-Twinax <full-duplex,rxpause,txpause>)
status: active
nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
plugged: SFP/SFP+/SFP28 1X Copper Active (Copper pigtail)
vendor: BROCADE PN: 58-1000026-01 SN: CAX116410001093 DATE: 2016-10-07
Are you able to test a different cable? I do have a lower cost DAC cable here that throws errors at 10G.
Steve