Confirmed by MaotisjanPL
I think the case can be closed since after the release of 18w45 mobs started spawning again, even if extremely rarely but i believe that has already been reported
Singleplayer, sorry i forgot about that, tryashtar: no, for me hostile mobs don't spawn at ALL like in peaceful mode (and yes i checked the difficulty level [more than once])
No ressource packs or mods of any kind, shod have mention it, As for Screenshot, command received