I have defined a Conditional Formatting rule for a display of some Azure Subnet objects which uses the addressSpace.addressPrefixes attribute. This attribute is an array of strings holding the VNet addresses e.g.:
addressSpace:
addressPrefixes:
- "10.0.0.0/8"
- "172.16.1.0/24"
I then access this attribute for the Custom Hover Text thus:
=CONCATENATE('Space:' @"addressSpace"."addressPrefixes"[1])
In the above example this correctly results in hover text of "Space: 10.0.0.0/8" (also showing that your arrays are indexed from 1)
HOWEVER if the addressPrefixes array holds only one item I instead get "#ERROR".
It appears that the processing is unable to handle indexing an attribute with a single-valued array and in all cases where this array has only one value I get the error. Others with multiple values resolve correctly. Changing the index to [0] does not work.
This appears to me to be a bug in the processing.