Cannot get gear back from Tanlorn?

You are their Lydia, you're carrying their stuff.

There is an issue where you can't unequip an item that a Companion is wearing (double clicking on an equipment slot and dragging an item off an equipment slot sometimes don't work), but the workaround is to equip a different item to swap it out. Then you can take the item the Companion was wearing and put it on a different Companion. It's an annoying bug that only sometimes happens. Usually it's not an issue.

I have done further testing and found a way to reproduce this bug. It happens regardless of whether you have addons enabled. When you open a crafting interface (such as a Blacksmithing Station) you lose the ability to remove equipment from your Companions. This also occurs when you talk to a Deconstruction Assistant. I found that an easy way to fix this is to open your own Inventory screen. You don't need to equip or unequip anything from your own character; simply opening this menu will unlock the ability to unequip items from your Companions. @ZOS_Kevin