Hi,

I have been able to confirm this issue. I have been able to reproduce it in singleplayer as well.
The cause for this bug has been found, but I'm unable to say when it will be fixed.

To explain the bug in simple terms, the activation delay ("First Horde Delay") is actually the delay that occurs after the spawner is created before it starts working.
If you used the editor to place down a spawner module, this means that this delay is basically the delay since mission start (+3 secs for internal reasons).
Essentially, "First Horde Delay" does not take activation into account, which is of course a bit misleading.

I'm surprised we haven't discovered this problem earlier.

To workaround it, you would have to create a spawner module on the fly using scripts, though this requires some skill in scripting and that you dePBO the Z&D file to get the right module variable names.

This may be a bit much, so my advice to anyone would be to avoid this module for now if you want zombies to spawn on specific activation + a noticeable delay.
Use triggers and BIS_fnc_spawnGroup-alike functions instead.

Thank you for the update and response. This certainly explains what I was experiencing and my only guess at the time. I have no problem with doing a little scripting, just happy to know the source of my issues.