Put two clips on the main video track, lock the video and then delete the audio. Unlock the video and then drop any transition on the join. I can use the handles to make the transition shorter, but not longer. Having made it shorter, I can't even restore it to the orginal length using the handles....  but I can make the transition longer if I open the clip properties box and then enter a longer duration.

Do the same on the overlay track and there is no problem.

Can anyone think why this behaviour might be by design?

Same action here. You can still lengthen it by double clicking on the transition and then changing the duration.

I can't think of any reason you would ever want it to act this way.

Don