GET/PUT /agreements/{agreementId}/formFields setting readonly field

We have an automated process to create an agreement which is initially set to AUTHORING so we can modify all the readonly fields to true so when the pdf gets sent for signing all fields are disabled except for the signature field.

It works as expected but there was one instance that the pdf when got sent for signing the form fields stayed editable.

So we thought something went wrong in our code. We were able to trace the api calls from the logs for this particular document and it shows the call to PUT /agreements/{agreementId}/formFields to update the readonly field was actually successful. 

So far we can't replicate it, but since it happened we don't have gurantee that it won't happen again.

Any idea how to move forward?