There is an easy workaround for PLANAR surfaces. The bug comes from the fact that the “TweenSurfaces” tool fails at 90 degrees or greater, but works properly if the pair of input surfaces are inclined at less than 90 degrees to each other.
If you turn on the History recording and create a copy of one of the input surfaces, then rotate it to become closer to the opposite surface (to reach the less than 90 degrees angle), you can build a median (tween surface), then rotate the copied surface back to its original position. The History will update the tween surface properly this time around. No idea why it works with this trick, but fails with the basic command.
BUG - Tween surface epic fail - PLUS FIX.3dm (4.5 MB)
