Not able to write to disabled fields on save

Spent hours (actually days) troubleshooting this bizarre behavior.

I have sourced fields on a custom record form for Customer (entity), Transaction, and Item. My client-side script code disables all three fields when they are all set (using nlapiDisableField) so they can not be changed. When I save the form, only the Customer field contained data, the other two fields were null.

Interestingly, I popped up an alert in my SaveRecord event to see what the fields contained which they were correctly populated with data. I also created a server side script to log what the fields had for data and they showed them as null so clearly the data was being wiped out somewhere after my SaveRecord event occurred and before the 'BeforeSubmit' event was called on the server side.