I had trade data, otherwise there would be no trade arrow on the galaxy map, right?
The galaxy map continued to show me the incorrect trade arrow while I was actually docked at the only station in the system.
Not necessarily. If I have trade data for a neighbouring system (call it "A") it might show system "B" as a source of a given commodity, so there would be an arrow in the galaxy map from B -> A. But then I found if you buy trade data for "B", you often find arrows for that commodity going into "B" as well.
If you want to be certain a system is a source of that commodity, I think it's best to make sure you've bought trade data for that system and there are no incoming arrows. Don't just rely on one or more outgoing arrows.
(Note - there definitely could be bugs! But countless times I was caught out by the above, and the tip above seems to make it much more reliable).