There is a known issue with doing a transition between two clips based on the same AVCHD source file. As you have verified, the issue doesn't occur between two clips that are not from the same asset (source file).
If you need to apply a transition between two clips from the same asset, the workaround is to duplicate the file and use one clip from each copy on either side of the transition.
Is this only for editing preview, or does it also not work in the export?
Were you asking me, Jim? The known issue just affects the smoothness of previews; it shouldn't affect rendered output.
Thanks Todd... I had searched around and was not successful in finding a similar post/response to this issue. I can verify (per Jim's post) that when I did a test render, the effect/transition was fine in actual render... so only previews are affected.
Thanks for posting the workaround. Given transitions are generally short, I thought I might be able to razor/cut shortly before and after it the transition... so the preview only rendered a short segment, but that does not work... it still renders the whole segment (for anyone that had this idea too).
If there are any other issues of this type to know of, I would appreciate if you can post it here as a follow up it would be greatly appreciated.
Thanks again, J