directly connected links - one side not learning MAC Address, wireshark shows "malformed packets"

ive installed a L3 switch (c9300), with two uplinks into different routers (juniper mx80). One link has come up fine, and i can ping across, the other link is behaving strange. It shows as "up", but i am unable to ping across. These are tenG interfaces both sides, with multimode fibre

Show ARP on the cisco switch lists the MAC address of the Juniper device, but show arp on the juniper side does not show the MAC of the Cisco device.

​

On top of that, i have carried out a wireshark capture, when pinging from the mx80, i can see the juniper arp request, and the cisco device responds to it with no issues. when pinging from the cisco switch i just see echo requests, as obviously it has the MAC but i dont get a response (not even an ARP request from the juniper side)

On top of that. If i clear the arp entries on the cisco side, and then ping across, i do not get a response from the Juniper side, so i dont get an arp entry. If i then ping from the juniper side i get an arp entry in the cisco switch

Any tips on fault finding this issue?