For the most part, the side input of a comparator is powered only by a certain kind of redstone input... directional power. Redstone wire, a repeater, or another comparator all qualify this description. However, ever since 15w46a, redstone blocks have also powered the side input of a comparator. This is inconsistent for a number of reasons:

• Redstone blocks are not directional power sources like the other things that power the side input.
• Redstone blocks emit activation power (like the side of a redstone torch or non-attached side of a button/lever/pressure plate), whereas the other things that are accepted by a comparator's side input emit either weak power (redstone wire) or strong power (repeater & comparator).
• If redstone blocks can power the side input of a comparator, then so should buttons, levers, pressure plates, redstone torches, & trapped chests. But they don't.

There are 2 solutions to this bug:

• Solution A: Revert the comparator side input behavior to what it was pre-15w46a, because the old behavior was far more consistent.
• Solution B: Go the opposite direction and make all activation power sources power the side input of comparators.

If you go with solution B, keep in mind that this option may break some redstone contraptions... far more than solution A, which probably wouldn't break anything, since almost no one ever uses a redstone block next to a comparator to power the side input. In fact, solution A might actually fix some old builds that got broken by the 15w46a change.