Possible CRM bug: Business rules overrule Read-only records

My colleague Niclas Lund Stisager ran into a scenario where a customer was able to edit fields on a closed opportunity, because a business rule unlocked a field for a different purpose. Even after the record was closed and read-only, the business rule overrides the read-only on the form, and allows the user to update the fields, which are controlled by the business rule. I’ve tested this on CRM 2015 On-Prem, CRM 2016 On-Prem, CRM 2015 Online v7.1 and CRM 2016 Online v8.0, and it is present in all versions.

If we take a closed opportunity record, the fields are of cause in read-only state.

If we then add a business rule, and then add an unlock action

Then we will be able to open up the Optionset on the closed record, and save the change

There is an easy fix to this, which is just to add another criteria on the business rule. This does not feel like an intended behavior.