what is the usual reason for drop on a fortiswitch

greetings all,

here is the topology. let's focus on the left part. MPLS router is a Cisco router, the left L2 switch connected to MPLS router is FortiSwitch (108F).

we occasionally lost MPLS connection, and we did see there are some input drops on port 2 (blue arrow pointing to it in the above diagram) of the FortiSwitch (left  L2 switch in the above diagram). see below screenshot.

vendor engineer recommended to replace the cable between the mpls router and the left L2 switch. Yes, of course we can do it, but may I know what's the common reason for input drops. MTU on MPLS router is 1500 by default, I don't think this is the problem.

Thanks a lot!