Just wanted to share this, as I don't think anyone else has had this experience.

I cast Mordenkainen's Faithful Hound during my fight with Hedrack. After the fight I teleported back to Nulb for rest and recovery rather than backtracking out of the temple. And lo! The hound stayed with me even after I rested. I don't know what caused the bug, whether the spell itself is bugged or if the combination of the spell plus the teleport spell did it. But regardless, it's now a semi-controllable party member, like an animal companion. Given that the hound cannot be attacked, I now have a permanent unkillable animal companion! Granted, it's not that great with only +10 attack and it lost its invisibility, but it's still a pretty neat addition. A truly faithful hound indeed!

I tested the spell again tonight and it seems like the spell itself is bugged. Casted it and rested a day until the spell expired, and the second hound stayed with me just like the first. You can make an army of invincible hounds like this!

This isn't the only bug in the game.

You can stack False Life spells to get extremely large amounts of temporary hit points, turning Sorcerers into really good tanks. (If you do this, get Extend Spell so that you can cast long-lasting False Life spells before you rest and still have them running afterwords.)

Unfortunately, this doesn't work with Vampiric Touch because that spell does not function properly; it does the wrong amount of damage and gives you 0 temporary hit points.