Imported Constant curve interpolates over 2 frames instead of only 1.

This is a regression from 5.2 to 5.3
not repro in 5.2
repro in 5.3

oldest version tested with the bug is...
Version 5.3.0a1 (6b90fd995f18)
Wed, 26 Aug 2015 15:45:29 GMT
Branch: trunk

most recent version with this bug is...
Version 5.4.0a3 (0129bfc37dad)
Sat, 14 Nov 2015 01:18:41 GMT
Branch: 5.4/animation/dev

Repro:
Add attached Interpolations.fbx to project.
Drop Interpolations asset into scene.
Grab "Take 001" clip under Interpolations, and drop it onto Interpolations GO in the scene.
Select "Stepped : Position" transform and step frame by frame through frame 30 to see the Stepped GO takes 2 frames to step from previous to next value... should only take 1 frame.
frame 29 = good
frame 30 = bad, only half value
frame 31 = good

notes:
This only happens when "importing" constant cuves, created constant curves are fine. This happens on any transform, not only rotations. This happens regardless of the "Resample Rotation" import option.