I have a slew of torches set up (approx 250 blocks by 250 blocks) around my base, all of them 12 blocks apart, with alternating rows that are offset by 6 blocks (with additional torches set up for height variance causing light levels to drop further). This has always been how I torch around my base to avoid all hostile mobs spawning.

I became very confused when I kept finding hostile mobs around my base still. Especially spiders. Lots of spiders.

After sitting and watching I noticed that they're always starting at the top of trees I have around my base. I have every tree type set up in a circle around my base and have seen mobs in all of them so it's not any specific type of tree.

I also have an oak tree farm set up with 32 blocks of consecutive trees, row of torches, then another row of oak trees (this setup is 12 blocks wide). After I saw two skeletons at the very top of this tree farm, I realized exactly what was happening.

Hostile mobs are spawning on leaf blocks.

I've validated that there are no exposed wood blocks at the top of the trees that they might be spawning on. They are absolutely spawning on top of leaves.

Mobs are also spawning at the top of mushroom trees, but I'm not sure if this has always been the case and whether it is expected or not.

To reproduce, I'd recommend setting up a tree farm and just waiting to see:

(T's are trees H's are torches)

H H H H H ...
T T T T T T ...
.H H H H H ...
T T T T T T ...

(see attached screenshot for two spiders on top of tree farm)