Well, I created a new Zone Object from scratch and copied the script from the 'corrupted' one, and now everything seems to be OK (unfortunately, the new part no longer has the same GUID). What's also odd is the script was apparently using the 'switched' parameter values and I was getting various error messages (e.g. "string type expression required") when there was no apparent error to the script.
MacBook Pro Apple M2 Max, 96 GB of RAM
AC27 US (5003) on Mac OS Ventura 13.6.2
Started on AC4.0 in 91/92/93; full-time user since AC8.1 in 2004