I think it's very unusual.
I do probably half a dozen, even as a fairly light rail user, maybe ~75 legs per year, mainly of ToD non-advance tickets for business trips that I no longer need or have to change, or the timetable has changed, or it's a strike day. Always simple point to point walk-up tickets to London Terminals or an airport.
The Thameslink site makes this extremely easy when logged in and completely free so I tend to use this for simple journeys where I want peace of mind of getting my bookings all in place early, but that there is any possibility of cancellation. It also issues an itinerary on Travelcard and LU destinations which not all do. I use Trainsplit for the majority but anything that
might be cancelled gets booked through TL.
I also booked a duplicate having forgotten I had booked it already when planning the trip, and that was uncollected and refunded seamlessly even after the date of travel. I never collected the ticket so it was a few clicks and done.
But I fulfil more than I refund, even with Thameslink, my "refundable" choice of booking site, and even with half a dozen bookings cancelled out of maybe 20 a year, I would be surprised any algorithm would be that sensitive. And there are no irregularities in my history so would never expect a letter even if flagged.
Refunding almost everything is another matter...