Enderman Teleportation Problem

so, I have a friendly ender man (Oleg), who I met today. I put him in a minecart, and he teleported away! Luckily, I was able to secure him and put him in a chamber, but he might not stay there for a long time. I thought that he was supposed to stay in the minecart/boat, but apparently not. Is this intentional, or is it just a feature that was overlooked? I don't want to put him down, but I might have to if this problem keeps happening.