Mobs do not spawn from upgraded superflat worlds

Mobs are not spawning in my superflat worlds that have been upgraded to 1.14 from 1.11. Before upgrading, the command "/gamerule doMobSpawning false" was active, but I have, in 1.14, updated this to allow mob spawning, but no change is visible. Tested on multiple 1.11 superflat worlds that have been updated to 1.14