And see, I think that's too broad, because literally everyone is guilty of holding onto a belief that they formed before they had enough information, however small.

Have you ever driven one route from point A to B, but taken a completely different route from B to A, both directions believing you are taking the fastest route? Maybe it's doublethink, maybe we just got in a habit and never reconciled the conflicting beliefs, or maybe we think the evidence we've been presented with is not a representative sample of reality. Maybe a map shows one route to be obviously faster both ways, but you think "well once you factor in the lights, and the number of turns, and the traffic at the times of day I take each route, it makes sense to take different routes each way. These are hard to account for on a map, and how I do it feels shorter, so I'm going to keep doing what I think is best regardless of what this data says."

To me, the "defining feature" of a flat earther is accepting a false belief after it's been amply demonstrated to you to be false. It's not something you didn't have enough evidence about, but now you do, it's something you had overwhelming evidence for, but reject it all. That is not something we all do every day, that is potentially delusional behavior.