We recently made a change to our hardware setup to increase the total number of monsters spawned. This change had the side effect of also causing initial monster spawns after maintenance to take slightly longer, but as maintenance is a block of time, extra time to start spawns seemed like a reasonable trade off. Unfortunately we stumbled upon an ancient bug where if a monster takes too long to spawn, it effectively breaks the spawn region. This problem never occurred on the development server or Blight, but with Chaos and the different complexity of the database, the extra bit of time tipped us over a threshold and some monsters failed.
Now that we've identified what's causing the bug we will be working on additional fixes. All these changes are part of a bigger set of changes so that we can support future content. We're confident we'll be able to get a fix in place and please be assured we're working non-stop on this problem.
We'll update you when more information is available (we're trying to keep the number of mentions to a minimum). We've also implemented more fixes for the disappearing NPC problem, and we're confident that once this troubled time is over, things will be much better on Istaria's shards. Thank you very much for being so understanding and we will not rest until things are working properly again.
Istaria Development Team