Xantech tech support have actually been very good and are helping me resolve this issue. All the troubleshooting steps that I referred to in my original post (i.e. wire testing, changing zones with same SPLCDs and vice versa etc) were suggested by them – many of these troubleshooting steps had already been completed by me.

They also suggested ensuring that latest software and firmware versions were loaded in MRC and SPLCD units. They have also looked at the project that was loaded on these and found nothing wrong – yet. At this point, they do not know what is causing these issues but are looking into it.

I will probably end up sending a SPLCD unit back to them so that they can test it in their environment, but I really wanted to make sure that there was nothing wrong that I was doing on my end first.