Can't modify field in a business process

I have 2 identical fields on my entity (they never show up both in a same time). But I can't update 1 of them in my business process. I can modify it when creating or updating record but can't do it in workflow. I disabled "read only" mode of the controller, disabled security role and onload script but still this field is unreachable for me. Another one identical field can be modified. This trouble appears both in Dynamics 2011 and 365 and I don't know what to do.