The RS485 bus is particularly sensitive. When I use it, and I do, I take many wiring precautions and have the network terminated with an RC impedance, not just R.

Without going into too much details, your experience is reminiscent of a similar case that happened to me with other devices, where some nodes would go offline due to too many framing errors, then requiring the node to be rebooted. I diagnosed on the network the presence of differential mode disturbances caused by other device working only on spot and, therefore, I noticed it only fortunately after long investigation.

I don't know if this is your case, but it could be something similar. Then, if as you say other devices instead always work fine, it could also be defectiveness of the affected one.
Start by trying to figure out, if it exists, through a errors log of the node goes offline, otherwise investigate the state of the network a little bit, at the cost of putting a PC at one end and with an analyzer log and check the packets passing through the netwrk. Also check the modbus network from the electrical points: impedance, both by measuring it and by checking for the absence of reflected signal, any presence of common mode noise at the various points where the devices are installed, any presence of differential mode noise perhaps caused by other specific devices.

Good luck.