onslaught-1.12.2-1.2.4.jar playing a modpack:
https://www.curseforge.com/minecraft/modpacks/ancient-warfare-horizons

Had a zombie invasion; could NOT find the last one. He lingered for about a week in game. I tried quitting, rebooting, etc, but the zombie would never teleport to me, no matter how far I traveled away from the initial invasion spot. Finally switched my game to peaceful, then back to normal, and it teleported the invading zombie near me. Just posting this in case it helps. Awesome invasion mod.

Any chance of having invaders pillar? If so, someone made a great pillaring block (designed to disappear in sunlight):
https://www.curseforge.com/minecraft/mc-mods/vanishment-point

I often experience something like this but maybe I'm also doing something wrong with the config.