I have since left the system, and returned by warping back into the system, rather than a station teleport.
Upon entering the system, the planet was marked as unmapped. When I landed on it for the second time this evening, it's data all updated and I was able to rename it again, and re-upload it's data.
So it seems that you have to re-visit a planet twice to be able to update it's discovery data. I will visit the two other planets in the system and report my findings.
So this must be a bug or an anomaly as the second planet said I'd already discovered it and it's data was the same.