This problem happens only when SFP module is inserted - probably because then ethernet ports share all the same lane and the SFP gets its own private lane to the CPU.
After some time (could be days or weeks, sometimes only hours) other interfaces stop working - RB sees ARP table, but I don't have ping or even arping to those hosts connected to other interfaces.
If i do some operation (like disable/enable) on those interfaces, all ARP (on other than SFP interfaces) get lost and no longer appears.
Only reboot fixes it.
This problem appears on several RB760iGS (probably all that use SFP).
I created support ticket more than a month ago, attached 2 supout.rif files - you think I got ANY reaction? :(

Is the SFP module a compatible one ?
Network diagram ?

The only way to work near stable on sfp1 on hEX s is:
do not support non-half and non-1G speed on any sfp compatible modules

Is the SFP module a compatible one ?
Network diagram ?

It should be, according to the wiki:

MikroTik devices and SFP/SFP+/QSFP+ modules do not have any restrictions for other vendor equipment. As long as the other vendor modules and devices comply with transceiver multi-source agreement (MSA) they should be compatible with MikroTik.

it's common "Cisco compatible" SFP that we use on hundreds of other devices without any problem, even other Routerboard models
but I can try using Mikrotik SFP if it makes any difference...

Diagram is simple, something like this:
Juniper swith --- EdgeCore switch --- sfp1 on RB760iGS = WAN (172.17.21.5/24)
Bridge (ether1 - ether5) = LAN (/29 public IP routed via WAN using OSPF/BGP)
(or even without bridge, doesn't make a difference)

The only way to work near stable on sfp1 on hEX s is:

well connection is stable as far as sfp1 goes - but other ethernet ports get locked, I can still access RB via SFP when that happens
and we use only 1Gbit FD modules

What SFP module you use?

What SFP module you use?

GigaLight 1.25G SM 1310/1550 (so like S-35/53LC20D)

I have the same problem with two RB760iGS.
It's is strange because I have totally six RB760iGS. And problems are observed just on two of them.
And I can not understand what is difference between routers which operate well and ones with problem.

Anyone find a solution?

I have fifteen RB760 in my network, they are all working fine using the SFP interface until some days ago with firmware 6.48.6.
Then, suddently a bounch of them start to showing the exactly same behavior that you describe.
We upgraded the firmware for the 6.49.10 and the problems persist.

Anyone expirience this behavior with RouterOS V7 ?

I have the same problem. I have tried from v6 to v7.

V7 has more CPU resources
I do not recommend it with the SFP port installed until Mikrotik helps us with a version that fixes it.