Villagers and Mobs disappear after a while when playing in Superflat. Hostile Mobs, Bats and Iron Golems stay, sometimes cats too.
*I tried creating Superflat in 1.15.2 and it worked there without bugging, then I switched back to 20w12a and the Villagers disappeared again after I left the closest Village I spawned to.
They do *not reappear when switching back to 1.15.2 and playing the same world there.
*At first there is a normal amount of Villagers, not only 1 to 3 from the beginning on like in other bug reports.