Sure, this is the source: Sanctuary of the Serpent - Official Conan Exiles Wiki
But to me, it also feels like 10%, to be honest. But since I never have much luck in games, I assumed, others might find more.

This is a very common argument and if you look at the single runs it is true. But statistically numbers even out and if you take a high number, like one million runs, for example, roughly 25% should have given a recipe - meaning a quarter, meaning every fourth.

Sorry, the number is probably a Chat-GPT error, which I used to calculate the chances and which (after some experimenting) apparently sucks at it. Using the “Coupon Collecter Formula” you should have to find an average of 166 recipes to get a full set. If we assume only every fourth chest is a recipe, you have to open an average of 664 chests to get all.

While the number is significantly lower than the one I stated in the first posting, I still think it is unreasonably high. Because if you want Grand Master Armorsmith and Grand Master Weaponsmith in Exiled Lands, you go Warmaker just one single time and get both.