When a repeater faces a block, the block becomes powered but does not update. Because of this, things that are dependent on the block's power (torches, dispensers, etc) do not update properly. For some reason, TNT still ignites, however.

Examples include item sorters, item elevators, and so on. See the video at https://youtu.be/ZhXXcU9-uhs

I would imagine this bug is caused in attempt to limit the capabilities of the observer block. Because the observer block shouldn't be able to detect redstone updates to the block, the feature of redstone updating blocks was simply removed. However, this also causes errors when updating other items attached to the block, such as, in this case, torches.