Serial nodes appear to stop working

I have a dozen or so devices in the field that run NR for months on end without a restart. The application makes heavy use of the serial nodes, with several messages per second crossing the wire. I'm running NR 1.0.x with Debian Buster on Raspberry Pi 3B / B+.

I'm starting to see devices that have 3-4 months of uptime lose their ability to communicate over the serial port. This has happened four times, each time on a different device. The symptom is that the device being controlled on the other side of the line no longer responds to commands. There are many possible reasons for this, I realize. However, the cure is simply to reboot the pi (but not the device it is communicating with, notably).

I would love to know the actual cause, but it's difficult to get access to the devices. I have not succeeded in recreating the problem in my own environment. Has anyone else seen this issue with the serial nodes?