Could you try re-logging to PRPC after creating new ruleset version or may be re-saving application rule? I remember facing a similar issue with Pega 7 where I have done one of these after creating a new version, but cannot recall exact scenario.
I seem to be on to something. I ran a report definition on Rule-RuleSet-Name listing .pyRuleSetName and .pyNoOpenRuleSetForCustomFields. Ignoring the guardrail warning (unoptimized property), I ran the report.
For all rulesets, the .pyNoOpenRuleSetForCustomFields is empty EXCEPT for the problematic ruleset. It is the only one with the value .pyNoOpenRuleSetForCustomFields = false.