Under normal circumstances, a powered rail on a block will power redstone under that block when a mine cart goes over it. After passing the detector rail the redstone un-powers.

Well, in this case, the redstone is staying powered.

If the detector rail is placed on the block first, when the redstone is added under the block supporting the rail it becomes powered.

However, if the redstone is placed before the detector rail it will be un-powered until either a cart passes over the detector rail, or the redstone is updated, via block placed next to it, etc.

Note
I have been able to recreate the same bug on new generated worlds as well.
so it isn't world specific.

It is very simple to recreate. Just 2 solid blocks, a detector rail, and a piece of redstone.