Qatar seat fee refunds after flight? Is this normal?

I purchased Perth to London return flights, and paid extra for preferred seating in the upper deck economy section on the A380 for all 4 legs.

Between purchase and departure, 3 of the 4 legs were changed from the A380, and each time I was refunded the preferred seating fee, and I then purchased preferred seats on the new aircraft. Annoying, but I still had preferred seats so no problem.

As soon as I landed in Doha, I received a refund notification saying my preferred seat was no longer available due to an aircraft change. I still had my preferred seat for the connecting flight to London, and the man at the help desk couldn’t explain the refund. After landing in London, again I received a refund notification. Then after returning home, the same thing happened and I got a refund for the last leg.

I’m not complaining as I had over 700AUD returned, but can anyone explain why this happened?