Troubleshooting documentExtractError

Hi all, I'm trying to get to the bottom of an issue that is occurring with some of our PDF uploads (using multipart/form-data). From the response we see:

{"code":400,"message":"[error.validation.verifyDocument.documentExtractError]","messageKey":"error.validation.verifyDocument.documentExtractError","name":"Validation Error"}

And in the server logs we see:

2019-02-19 11:55:26.677 [20819]ERROR  : Could not find search string [SIGNATURE OF CONTRACTING OFFICER].
2019-02-19 11:55:26.677 [20819]ERROR  : Command [ExtractMultiTextAnchor (file: /tmp/20190219/ffd0fee0-033b-412f-b717-30f493a4dd85-5711-
...
[2/19/19 11:55:26:733 CST] 000002a8 OperationResp W   Operation update returned error error.eslx.inputValidation.documentExtractError

So my guess is, inside the PDF there is a signature object "SIGNATURE OF CONTRACTING OFFICER" which is declared somewhere and not not found, thus throwing the error. I'm told that if we use the exact same JSON and PDF and load it via Postman it works fine. I'm going to attempt to reproduce the error inside my sandbox account, but was hoping that your trained eye will be able to spot something obvious and lead me into the right place. I appreciate any help. Thank you!