The Texture of Dropped/Ignited TnT is sideways, Not only TnT but all dropped Items/Block such as Logs, Pillars, Haybales, and etc.

The Side Texture of all dropped/activated blocks land Its Opposite side are sideways

Steps to reproduce bug (Activated Block/Item)
1. Place a TNT
2. Ignite it with Flint & Steel or Lava and etc. (you will observed that its side texture is sideways)

Steps to reproduce bug (Dropped Block/Item)
1. Get any block or item
2. Drop it or Throw it (you will observed that its side texture is sideways