I just recently encountered the same issue with the pets myself this past week. I was rolling through Sol Ro tower with my warrior and mage doing the quest.
At first I thought I was doing to much damage to Rizlona on my warrior and that my mage couldn't handle the pets fast enough once I started killing them, yes pet hold and focus was turned on. You will start to see the yellow system text warning you of to many NPC's once you have 21 on your aggro list, after 25 you get ported.
I can't confirm if you wipe that you have to wait an entire 24 hours to try again, but if you get ported out because of to many NPC's that yes you have to wait 24 hours for Rizlona to respawn. I don't know of any other encounter that despawns for 24 hours if you wipe, especially if you get ejected from the zone. Kinda lame.
Also, the adds do not seem to be % based but on a timer. The next day I went back with my warrior and mage and tried the fight again. Once the first pair of pets spawned, I turned off attack on the warrior and the mage with pet hold and focus enabled again I started taking down the pets once more... except another pair of pets spawned while my mage pet was killing Rizlonda's second pet from the first pair. So... I proceeded to burn down the dragon as fast as possible and managed to survive with 24 NPC's on my aggro list. If the adds were % based I should have been ejected before Rizlona died. It's not like a donor weapon/aug rk 2 water pet can one shot that dragon... my deeps isn't that crazy good to consider I did an extensive amount of damage in that I barely managed to survive the second time around. Once I saw the second pair of pets get pooped out, I knew it couldn't be % based if I was doing zero damage to the boss.
Good luck to others trying this event.