How to prevent Actors being deleted at coordinates of +/-262,101+?

Because of the way floating point values work in computers, going too far away from the origin (too far away from 0) introduces instability and inaccuracy. This is a hardware limitation and isn’t going to change. (You can use double precision, which moves the problem further away, but there’s still a limit somewhere.)

So, what you have to do when you build an “infinite” world, is to build smaller sub-worlds, each of which is centered on 0,0,0, and then “jump” the player (and other actors) relative to the world when you switch from one world to the next.

There is actually support for this already built into Unreal Engine: Unreal Engine 4 - Introduction to Level Streaming - YouTube