hope that the mercenary camp has loading time when you choose a mercenary. i think a lot of players mistakenly click the wrong mercenary while scrolling at the barrack cause the lag while transfering from barracks to mercenary camp.
I've never experienced this issue myself. Almost all of my mercenary recruitment has been through camps discovered from clearing forests, adding them to my own as I increase capacity (and after they, ah, don't return from battle shall we say).