We have a couple of RB750GL's running RouterOS 5.5 connected via ether1.
They will not pass packets greater than 1500bytes between them, even with an MTU of 1598 on eth1 they will only pass up to 1500byte packetsOddly, RouterOS thinks the packet was sent successfully as there are no fragmentation errors, the packet just never leaves the port!
I have logged a ticket with Mikrotik, so hopefully someone gets on to it before anyone else encounters this problem.