Do merchants keep the items you sell to them forever?

I couldn't really find a consensus among players when googling this topic so I decided to do some testing.

After completing the last main quests of the Blood & Wine expansion I noticed that Lafargue's stock had reset! This made me think that either:

1. Completing the quest(s) had reset his stock just because, or;
2. Time passes after completing those quests so maybe it had nothing to do with those quests and it was just because some time had passed.

I loaded a save file just before seeing Damien in the Capture the Castle quest (which would lock me in to the game's final quests) and I checked Lafargue's stock. He still has all the weapons I've sold to him. I then sold him a sword I had on hand at the time.

Meanwhile I went to the art merchant nearby and checked his stock. He only had his standard set of items and didn't retain any of the junk I've sold to him in the past. I then sold him some more junk I had on hand.

I then meditated for 5 days and checked on the art merchant and Lafargue. The art merchant's stock had reset as expected, however with Lafargue only the sword I sold to him last was gone and he still retained all the old weapons. I meditated for an additional 30 days or so just to make sure and it didn't make any difference.

So in conclusion:

• It seems that the stock should (and does) reset, but for some reason Lafargue's stock didn't. If I sell him a sword then meditate 5 days only that sword will disappear but not the other items?
• Finishing the final B&W main quests did reset Lafargue back to normal.
• So a glitch then? Or maybe there was some period of time where the game on purpose retains all stuff sold to him, but I can't think of why this would be the case.

Edit: You can download my PS4 save files here (GOTY v1.60).