Visual glitch involving redstone

There are a few distinct definitions of a "solid block".

The most common is whether it will conduct redstone power and block a sloping signal. TNT doesn't do this. Before 1.8, this definition also exclusively determined whether players could suffocate inside the block, so TNT was used commonly in X-ray devices.

There is also the definition of whether a block visually occupies the entire cube, which is used by the renderer to decide if the top wire should connect to the bottom, or be isolated.

Glowstone, TNT and Blocks of Redstone are blocks which satisfy the second definition but not the first.

Ah yes, it does not occur to me that TNT is slightly smaller than a full block, and i have before seen this visual glitch in glowstone redstone towers. Despite this, should the wire not activate the TNT? Given that TNT behaves in this transparent manner, let's look at pistons. I have seen many redstone devices that incorporate the design I've uploaded where the TNT is replaced with an upward facing piston. The TNT and the piston are both implicitly transparent. They both allow passage of redstone. Naturally, pistons being transparent means the redstone visually updates to pass through and under it, but being defined as opaque, TNT doesn't allow the redstone to visually connect. In terms of coding of redstone, they are doing the exact same thing to the piston and TNT, but only the piston gets powered. The TNT should, don't you think?

It seems to be a quirk that allows redstone wire to point asymmetrically (see piston_extension.png).

If you replace the lime wool with redstone, the piston/TNT turns on.

I assume you are correct - I replaced the lime wool that you suggested replacing with various blocks. I found:
Placing a device activated by redstone, but does not interfere with it's orientation (e.g. wooden door) Activated, but the piston/TNT didn't.
Placing a device that interacts with and changes redstone's orientation (e.g. button, pressure plate, redstone) meant the piston/TNT did activate.

• These orientation devices explicitly orientated the redstone signal toward the piston/TNT extending it. I assume this confirms your findings, and confirms this as a visual bug?

Furthermore, I found that this is not orientation based (doesn't even follow South-East rule), which confuses me - so after further testing of orientation based signals, my findings seem to show that the redstone only has one direction - let me explain:

• When a dust is placed and isn't connected to anything, it powers in 4 directions on the x,z axis, North;East;South;West, or NESW respectively.

• When a single device is placed adjacent to redstone, redstone is orientated in two directions, toward and away from the device.
  (e.g. A device placed North of redstone will make the redstone point N = 'Toward' and S = 'Away')

• When two devices are placed adjacent to the redstone, it is orientated in two directions, but with only toward power.
  (e.g. As above, with another device to the East will make the redstone N = 'Toward', S = 'Away' and E = 'Toward')

• From here on until 4 devices, each device adds one direction. 4 devices means 4 directions.

• Note that a single direction is impossible, only two or more can exist at a time.
  Linking back to my point of confusion above, this redstone only seems to have one direction, as it is very clearly powering the lime wool, but not in the opposite direction (what should be its natural orientation; and to power a block, you need redstone to power in it's direction/orientation. I'm not too sure if this single directional behaviour was intended, but it doesn't seem logical or to conform with the laws of directional redstone anywhere else.