I would check the configs for Lycanites Mobs. Maybe there is something happening with that mod (or another you've added) that makes mobs invisible. If this were the case, an invisible creeper could seriously wreck your day. There may also be a config to disable mob griefing (I know one exists in configs, but maybe a special one for just that mod's mobs).
Maybe try disabling the mods you've added, then see if it happens. If not, add them back one at a time over the course of a week or so (your play schedule should dictate) until it begins again.
I know this doesn't happen in DW20 as it ships, so the problem is likely in those mods you added. Good luck.