We are facing the same issue. We need to store the original First Response case milestone target time. (First response only!) When it is updated (case: Stopped field is set to false), we need to to revert back to the original value.
We can get to the First Response milestone target time and save its original value, but unfortunately, we cannot revert it back (not with a workflow, PB, flow or trigger). I appreciate if this can be open for customization.