Comparator cake bug

I was testing the comparator to make my friends explode from tnt minecarts.

As far as I know, cakes are redstone signals to comparators.

To hide the trap, I wanted to give the comparator a redstone signal through an opaque block (the redstone signals arrives anyway).

After I ate the cake I noticed the comparator was still working without any redstone signal (I needed it to be off). The comparator still sends the output until you break the opaque block or update it (or just change the comparator mode subtract/compare).

I don't know if this is a bug, but it probably is, since it makes no sense.