Lan interface stops responding under high switch load on RB931

Hello all, I have a Mikrotik RB931-2nD "hAP mini" that is at the moment in use as a network switch only. This is the network configuration:

config interface 'loopback'
        option ifname 'lo'
        option proto 'static'
        option ipaddr ''
        option netmask ''

config interface 'lan'
        option type 'bridge'
        option ifname 'eth0'
        option proto 'dhcp'
        option ip6assign '60'

config switch
        option name 'switch0'
        option reset '1'
        option enable_vlan '1'
        option ports '1 2 3'

This works well as long as the network switch is only lightly loaded (browsing the web, streaming radio, etc...). However when the network switch is heavily loaded (anything more than a few megabits per second) like during a file copy, the "lan" interface of the RB931 stops responding. When doing a ping to the IP address of the "lan" interface, it works until I start copying a file, then the ping drops out until the file copy completes.
It's like the network switch has total and complete priority over the operating system's "lan" interface, and when 2 ports in the network switch are carrying a lot of traffic, everything coming from the OS "lan" interface is simply dropped.
There is nothing in the logs or dmesg when this happens. I've also tested this with iperf between 2 different hosts and the RB931 as switch in between the hosts, same thing happens. As soon as the RB931 network switch gets busy, the "lan" interface stops responding.

Can this be prevented somehow? The "lan" interface should always remain responsive, even under high switch load.