The summoning pendulum doesn't set the dimension tag to the current dimension when summoning the entity. This causes the entity to force the dimension it used to be in to load whenever its chunk is loaded. For reference as to testing done, please see the bottom of EnigmaticaModpacks/Enigmatica2Expert#703.

It looks like the Golden Lasso, which has similar functionality, solves this with the following line of code before actually spawning the entity:
entityTags.setInteger("Dimension", worldIn.provider.getDimension());

I hope this helps and I'd be happy to do any extra testing.