Tiara automatically topdecks everything including victory cards.
Do you have by chance set the corresponding autoplay option (shown below) to "Always topdeck"? Because that's the only way I can think this can happen automatically.
(https://i.imgur.com/fU3xwLWh.png)
You can reach these options and change them permanently by clicking on the gear icon at the top left of the screen, directly above the "Automatch" entry.
If you are in a game and want to change them for that game only, you can rightclick on the card (in your case Tiara) and chose the option that suits you best for now. For example, if you want to buy lots of low cost victory cards, (like Estates or Tunnel) it might be faster to change the autoplay to "Never topdeck", then buy them all without any interruption by the game and change the autoplay back to "off" afterwards.
Thanks that worked! I was trying the autoplay options, but I had initially thought that the setting was only for Royal Seal, rather than several cards.
Yes, we had to use one card for exposition, and the choice turns out not be that good now with Royal Seal depreciated.