We shouldn't be using $this, and Grahame believes there's a more straightforward expression structure that doesn't require substrings

I.e. "element.where(type.code='Reference' and id.endsWith('.reference') and type.targetProfile.exists() and id.substring(0,$this.length()-10) ..."

and ""element.where(type.code='CodeableConcept' and id.endsWith('.concept') and binding.exists() and id.substring(0,$this.length()-8)..."

This should ideally be handled as an R4B technical correction, but in the interim can be handled as a validator override.