I am not sure how the best way to trace this issue to find the problem but let me explain the steps.
A user creates an expense report and assigns it an expense category. Let's say they assign it to category General Supplies. When it goes into workflow review, the finance department reviews and the person reviewing it puts their initials into a field called internal note so that other users don't double up on the review. Nothing special about the note field. The expense report is rejected, sent back to the person doing the entry and they change the category to Office Supplies and resubmit it. To this point everything is working as expected. When the finance department gets it with the new category, they review it, everything is fine and they try to take off their initials in the internal note field and now even though in draft or review, the field is un-editable.
Additionally, not only is it un-editable but when you drill down on the transaction the field normally appears on the line item detail, it is now out of view.
An alternative way to try and edit the field is viewing the expense report, seeing the line items and clicking on edit without drilling down and still the field is not editable.
It looks like something in the category is what triggers it because if you do not change the category you can still modify the internal note field and it appears as though the re-direction to this particular account Office Supplies is what locks it down even if you submit the expense and go through the recall. So far, it doesn't sound like there are expense category changes and is due to a new policy to be more specific