During the last days, I had several cases of hostile mobs (including zombies, skeletons, creepers, spiders, even what I think are zombified villagers ...), appearing (spawning there or entering through closed doors, which they had never got through before) in well lit areas of my base underground.

One very characteristic case, which makes me think the problem is likely related to mobs spawning at a wrong light level, is that, today, a spider appeared in a well lit underground room (see attached screenshots), which only have 2 possible access ways, that, as far as I know, no spider should be able to pass through: a 1 block wide door, closed by an oaken door, a oaken trapdoor through the room ceiling, allowing access to a normally secured surface area...