CI4 Validation is_unique in properties Config/Validation

I've been struggling with exactly the same problem today. I have found no way to get the placeholder valu to work in the ruleset.

My solution has been to leave all the rules in the validation config file, then shortly before running the validation, I re-set the rule in my model by building the rule string with the actual value from the variable, by joining the strings as you have shown. This works.

I agree that it looks like a bug. I've left the placeholder in and if it gets fixed I can simply remove my extra ruleset code.