I've spent almost 10 hours in single player putting together a mob grinder and lighting up nearby caves. Until very recently the mob grinder was working perfectly - mobs would spawn and get sucked into the kill mechanism.
During the build I have been switching between "Peaceful" and "Hard" settings to let me build in peace.
Very recently the hostile mobs stopped spawning - I've got the setting on "Hard", have started and stopped the game, switched between Peaceful/Hard, and gone on long expeditions both overland and underground.
I have not seen or heard any hostile mobs but have come across neutral mobs (even chickens deep underground).
Has anyone experienced this? I've looked at this question: No hostile mobs spawning in minecraft - but it seems to only apply to multi-player/ Minecraft servers rather than single player.
Thanks in advance for any advice.