Mikrotik. Why do network connections hang?

Question

VPN

There is a vpn network for 3x 2011UAS + 1x 750UP.
RouterOS 6.7, current-firmware: 3.10.
Each of them has several uplinks (up to three) wrapped in PBR.
They are interconnected by ipip tunnels (mtu 1280), additionally clamp mtu-to-mss on the firewall, ipsec is turned off to be sure.
pps does not exceed 30k at speeds of the order of 60 Mbit/s.
qos and cos are off.
Internal routes propagated using ospf, bfd enabled - interval=0.2s min-rx=0.2s multiplier=5, all in one backbone.
After some time, the connections are "hanging" or torn, it is especially clearly visible on vnc and teamviewer - the picture simply stops updating. And they are torn not by timeout, but rather at moments when the amount of traffic increases.
The router logs are clean, no ospf and bfd breaks are observed.