It was reported on the Discord a while ago (https://discord.com/channels/751097183448727552/1206744411908603904), and this has also been my own experience. Attending somebody else's Mining Expedition does not allow you to gain the Miner trait or experience in that trait. Courtiers who accompany you on your expeditions also do not gain the trait. When I looked at the scripted effects, they all looked similar enough to the hunting equivalents, but those mostly work.
It was reported on the Discord a while ago (https://discord.com/channels/751097183448727552/1206744411908603904), and this has also been my own experience. Attending somebody else's Mining Expedition does not allow you to gain the Miner trait or experience in that trait. Courtiers who accompany you on your expeditions also do not gain the trait. When I looked at the scripted effects, they all looked similar enough to the hunting equivalents, but those mostly work.