Entities, such as name-tagged mobs, tamed mobs, villagers, mobs in boats/minecarts, said boats and minecarts, thrown tridents, etc. despawn upon reentering a previously existing world. Affects worlds on 1.17.30; not sure about any other version. Apparently affects worlds on other platforms, according to multiple complaints. Sorry if this has been reported already; I specifically created this account for reporting this bug and therefore have no experience on this site.