Add actions "Contact field is added" and "Contact field is removed" to work similar to the Step "Contact Tag Updated"
This way we can create a Workflow that starts with "Contact Field A is Added" and add a Step "Remove Field A Value" without generating a loop.
We should also consider adding a "Source" Trigger Filter, so that we only run the Workflow if the change of custom field came from a specific source. e.g. only Trigger if the field was changed manually by a user.