I have the same occurrence of rust monsters spawning midday in my game. If you open the drifter file and compare it to an older one, under spawning conditions the older file has listed MaxTimeOfDayLight as the light level restriction vs the newer files OnlyBlockLight rule. This may be why monsters are spawning all around as there is no torches around rifts to prevent spawns. Probably also why they're not really despawning as much
Shiver and Bowtorn files are the same result. Bell file is unaffected it seems