Switching

Display Type

from

Inline Text

to

Normal

has allowed the Suitelet to change it, but 1) I don't know why that would be the case and 2) that's not what I want

While the field is inline, the submitField is actually loading the whole record, setting the value and submitting the record. Once you changed it to disabled, the submitField is indeed submitting the field itself. Meaning, that your issues is with the load=> save part and it might be something to do with external script permissions and security restrictions.