Hostile mobs stop spawning.

VERSION 1.13.1  ---- NOT 1.13.6

Other bugs listed focus on only pigmen or the past bug of the game defaulting to peaceful. The latter is not happening for me and so is a different issue, perhaps related to the first, but not limited to pigmen.

In 1.13.1 on Realms, I have built a zombie pigmen farm and an overworld mob farm.

In both farms, mobs stop spawning whenever I am standing still. This is a crucial mechanic of the farms and as soon as I move around, they start spawning. I know that 20 min idle kick is a thing, which I negate by opening and closing a trapdoor, but even if i want to use the farm for 10 mins, it is impossible, as zero mobs spawn after the exiting ones die.

I have checked all of the spawn range, lighting and line of sight requirements for both designs; that is not the issue.