Ok this ones weird.
I have a project connected to an external piece of software, but I'm not sure if its relevant.
I have multiple date fields on this projects screen config, but on some tickets, a "Due" field appears with a prepopulated date. It's not the "Due Date" field and there is no custom field in the system called Due, so I feel like it's being triggered by same kind of calculation of the other date fields. The weirder thing is, there is no "Due" field on this project/issue types Screen Scheme. The field is not referenced at all in the history/activity section of the affected tickets. I'm leaning towards this has something to do with the external system it's connected to, but has anyone else ever seen this?