I was playing on my private server, building a witch farm, and a skeleton came out of a cave, so I re-logged, and while the skeleton did visually de-spawn, it still existed, and continued to shoot me, even though I couldn't see it, and it had no hitbox. (tested with F3+B.) After it died to sunlight, I could pick up its items. This has also happened with endermen (unfair), zombies and creepers. Please fix this. The server is vannila 1.13.2.
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This is an big issue. It's much worse when the zombies are in a cave, where no sunlight is present. The zombies are basically immutable and immortal. No way for them to die, especially if you've locked the difficulty or even worse play it on hardcore.