It appears that detector rails switch the direction of the rail before them (in relation to a moving minecart) before the detector rail is hit, under certain conditions. This is difficult to explain in words, so I have attached a photo diagram.

 Diagram:

Expected behavior:

When a chest or hopper minecart is placed on rail #1 (see diagram) and travels down the slope, I would expect rail #5 to switch directions to connect to rail #6 after the detector rail has been hit.

What actually happened:

When a chest or hopper minecart is placed on rail #1, rail #5 gets flipped when rail #4 is hit, and the detector rail appears to be in a powered state (see result photo below) before even being hit. Chest/hopper minecarts are then sent to rail 6 instead of the lower branch with the detector rail (so the detector rail never even gets hit)

Result photo:

Conditions:

• This does not happen if the chest or hopper minecart is placed on rail #2, it must be placed on rail #1
• This can also be reproduced with a slope of 5 normal rails, instead of 1 normal, 1 powered, 1 normal as shown in the photos. This leads me to believe that the chest/hopper minecart must hit a certain speed for this glitch to happen.
• Unable to reproduce this with furnace minecarts, tnt minecarts, or normal minecarts under these conditions (maybe needs more speed on the slope?)