Spider doesn't follow pathfinding route

Actual result:

The spider doesn't follow the predicted path. The behaviour only happens for some rotations of this test and not every time. 

Expected result:

The spider should reach the pathfinding target and never be able to ascend straight up with no connecting blocks.

Test steps:

1. Run: 

   /test run bigmobtests.altitude_change_wall_on_left_ascend_spider_rotation_1

1. Repeat to run the test until you get the error (less than 10 times should do it)
2. Note the spider behaviour.

Other:

See attached video for more information