The main issue is that now pets cannot be hurt or targeted, so they are no longer listed in the party interface. It is fairly easy to notice that an egi is gone, but your fairy? Sometimes in the heat of things, a tank pulling quickly, I only notice when I realize how .... damage is not being healed, then I have to waste a Swiftcast on a summons.

Pets are no longer healed, because pets no longer suffer damage. If they were listed in the party interface, you'd notice instantly. However, I can confirm that it happens when pets are on follow. If you run one of the dungeons which has you jumping off a waterfall or covering some other great distance dramatically, as the original poster mentioned, your pet will often exit.