Ticket inspector said ticket was showing up as ‘purchased after boarding’, but I purchased before I got to the station?

Yesterday I did my typical commute into London Paddington on a GWR train. I bought my ticket (an open return) on the way to the station via Trainline, and then used the QR code at the ticket barriers to get into the station successfully.

Coincidentally I’ve had a friend staying with me this week that did the same journey, at the same time as me on their way in to work - and also bought the same ticket from the Trainline, at approximately the same time as I did (to within 1 minute).

When I got on the train and the ticket inspector scanned my ticket about 5 mins into the journey, she said that I shouldn’t buy a ticket after departure as I could be fined. I explained I didn’t buy the ticket after departure- I bought it on the way to the station - but she said it was showing up on her system as ‘bought after departure’. Interestingly, my friend didn’t have this issue.

The only reason I can think this might have happened is that when we got to the station, we hopped on the first train to Paddington that was waiting at the station, which left the station about 4 mins later than it was scheduled to. However, I’m confused why the message didn’t appear for my friend but it did for me.

Anyone shed a light on why something like this might have happened?

Edit -

Thanks everyone for the responses! No conclusive answer, but as I expected I imagine it’s something to do with the fact I bought the ticket probably just after the original scheduled departure time of the train. The ticket inspector didn’t actually say that she would fine me, just that she could, but if I explained that I’d already used the ticket to get through the barrier then that should suffice as an explanation not to get fined. I think I’ll try buying my ticket at least 5 mins before I get to the station in future.