Hi,
I have the same problem, but I tried only on 2 versions.
1. I didn't try it on the new stable version downloaded in the morning automatically (sorry, I don't remember its version number).
2. The event page asked me to download an installer to see the new Black Friday event, so I installed it. I tried it on a flash sale, which changed back its price to the original, after it showed me an error message that the price had been changed.
3. I checked if there is a beta, but the client didn't find any, only an experimental, but it has the same bug for me. Its version is:
2.2.000 (Build 1014) / 64 bits / Qt 5.15.18
Downloader engine: 1.4.0.5
Edit: as I remember I had the same bug yesterday on the previous stable version. I just thought it is a display bug, but as I saw today, I purchase on the 100% price without flash sale (the triple diamond discount works, I'm buying a 30 cost show for 16.5 even on 20% fash sale (that should be 13.2)).