If you have a captivating treasure, 10 outlandish artifacts, and 15 heirlooms you can get a 17th heirloom by converting the treasure then the heirlooms without leaving your study. Presumably works the other way around and with 11 artifacts and the townhouse.
Very likely the check for how many heirlooms happens only when you enter your study and needs to happen when you create a heirloom. It’s an extremely minor bug\exploit.
The code in this game seems very tight TBQH, I’ve seen the very occasional oddity in fighting and getting double loot but I can’t replicate it. Failbetter does a good job in all aspects of what they do, esp the storytelling.