After device reboots routerOS doesn't install some static routes ( winbox shows the route in blue colour instead of black colour).
These routes are configured to be appllied only to specific packets via a routing mark.
RouterOS reinstalls the route if I disable and re-enable the ip address assigned to the exit interface used by the route itself.
Experiencing thus on different 4.x versions of routerOS.
Is it a bug ? Any workaround ?
Thanks.

Try to use an ip address as a gateway of the route instead of an interface name. Try to upgrade to 5.x if it's possible.