Summary:
It is well known in redstone, that redstone does not travel down from a slab to another block, only upwards. The screenshots below identifiy that the redstone dust on the slab turns to connect to the new piece of dust, i,e not powering the torch - however the torch is still on. Updating the torch with a block changes it's state.

Screenshots:
Grey blocks ==> Circuit design
Blue blocks ==> Steps to recreate problem
Fig. 1 - Redstone behaves as expected
Fig. 2 - Redstone stops powering the block. Visual glitch in the redstone?
Fig. 3 - Disproves that the redstone is visually glitched. Block placed by the torch updates it, showing that the torch should've been in a powered state, but wasn't.

Analysis:
Looking at the F3 debugger for the 3 screenshots, it is visible in the last two screenshots that the redstone has a South and East side, so the torch should be on in both of the last screenshots. But it isn't.