Can confirm this bug happens to me playing on windows 10 edition
This is also happening on realms. Probably a result of the fix for MCPE-50175, Which fixed a bug in the growth of kelp
this issue is fixed with the /tickingarea command. you can set huge areas to be always loaded in, as if there's a player there
I'm encountering this bug as well. I play on Windows 10 edition on a realm, but i have friends who play on switch on the realm and they get zombie pigman instead of zombified piglins as well. I'm playing on 1.16.0. Also they are definitely new spawns, we even have a gold farm that still spits out pigmen instead of piglins. I have a couple of screenshots from the game, one in the nether with new terrain in the background, and another from the gold farm.
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