perform the following setup within a single column of blocks 3 blocks tall in the Y direction, and 7 blocks long in the X or Z direction:

Activating the lever will cause the piston to activate.

Bug: when the lever is turned off the piston doesn't restract.

This length of 6 redstone is the first time this happens.  Redstone lengths of 1-5 all work properly.  Lengths 8, 10, 12, and 14 also don't work, but 7, 9, 11, 13, and 15 do.

Here's a 7-length example (just add one more block, and redstone, and move the piston down the row one block).

When the lever is placed at the same level as most of the redstone, the piston works for lengths, 6,8,10,etc.  Different patterns of having the redstone go up and down blocks yields different results regarding whether or not the piston will work.

This is probably one symptom of a much larger issue.