My recruited units don't vanish, but they stop appearing on the unit selection row in the upper left hand corner if they are without a champion or a regular created unit. I use the Beast Lord skill a lot, and in 1.5, half of the tamed creatures in a stack will be grayed out when you look at the stack. If you enter a city, the greyed out units vanish, meaning you can't pull them out of the city, but if that city is later attacked, they are there for the defense. if you let grayed out units on their own in the main map, they will not show up for selection as mentioned, but they are sitting there on the map and can be moved and used in combat.
I actually liked that mechanic in a way, it simulates the fact that you don't have COMPLETE control of ALL the creatures, with some preferring to stay in a city, or just hanging out in a tile if you don't keep track of them. The Beast Lord trait is pretty awesome with the amount of large spiders in a normal game. 4 Corpse spiders, 3 Harridans and two spell casters allowed me to take down Dellin and his force pretty easily, immobilizing them and using Sunder to kill them 130 hp at a clip.