I've face this issue on MikroTik CHR and CCR1072 with ROS 7.1.3, ROS 7.2, ROS 7.3.1.
When MikroTik has been using for a while (sometimes days, sometimes months) it will cannot ping to any devices (still can traceroute) so netwatch will malfunction but other modules are working fine.
Any solution to fix this? on CCR1072, I use schedule restart everyday and the problem do not happen again.
but on CHR, I use it as a production for 24x7 systems. Cannot just schedule restart. Everytime I want to reboot, I have to write a formal letter to notice several departments.