Save a Structure with a piston facing north and a repeater which powers it directly from the west side. When you load it, you will get a piston without a piston head.
Loading the Structure a second time (when you have a piston or a repeater at the same place) will place the piston head correctly.
Rotating or mirroring the structure has no effect.
Is this still an issue in the latest snapshot 16w44a? If so please update the affected versions.
This is an automated comment on any open or reopened issue with out-of-date affected versions.