Hi. I have sometimes observed this happening. Without any reason at all, the merc weapon gets unequipped and the insight or conviction aura goes away. I can lift up-> put down the merc weapon and then it comes back, but it is still very annoying.
It used to happen every time he died, but they fixed that in 1.12 iirc. It had gotten to be so habitual with me to lift up and drop the weapon back into place that I frequently find myself doing it still. I've noticed the equipping order still affecting auras however even if you don't need the stats. I've noticed the aura going off even if I just swap armors or hats (or when equipping any items in GoMule). You just need to make sure the aura giving item is last equipped (not sure how this would work with multiple aura granting items like a dream hat + insight).