Old One's Army T1 portal despawns for no reason

[Terashi]

For some reason, despite the fact that I'm killing mobs as soon as they spawn in, a weird wave effect would happen and the portal would disappear. I flattened a huge area but it kept happening. I right clicked the crystal shrine to make mobs spawn faster and got to wave 3 before the portal faded away again. It can't be a dps problem because I'm killing mobs the second they exit the portal, so I'm not sure what's wrong.

Here's my modlist in case somebody asks for it, listed in order of mods installed from newest to oldest just like Steam does. I also had Summoners Association and Start with Base but had them disabled, the former because I forgot to enable it and the latter because I wanted to focus on vanilla npc happiness.

Which Mod Is This From
The Aumlet Of Many Minions: Combat Pets!
Calamity Mod
Boss Checklist
Dialogue Panel Rework
The NPC Portraits Mod
Cheat Extensions for Hero's/Cheat Sheet
HERO's Mod
Subworld Library
The Stars Above
Shorter Respawn Time
Ore Exchange [broken]
Ore Excavator - The Successor to Vein Miner
Magic storage starter kit
Magic Storage
Recipe Browser && Magic Storage
Recipe Browser
Omniswing
AlchemistNPC Lite
Decadent Skills

I posted this in workshop discussions but got no responses there.