I can concur this happens. I have seen it with all beta versions of 15 (I am yet to install the release version)...
I am trying to see if there is a pattern to it, but essentially it is as simple as the OP advised. Placing the timeline-cursor in the middle of a clip, the following occurs: -
In Trim Mode
Shift [ - Trims clip left, ripples following clips to close gap, left transition preserved
Shift ] - Trims clip right, ripples following clips to close gap, right transition deleted
I actually use the following method, as it functions in both Selection and Trim modes, but the result is the same: -
Ctrl-Shift [ - Trims clip left, ripples following clips to close gap, left transition preserved
Ctrl-Shift ] - Trims clip right, ripples following clips to close gap, right transition deleted
Hope that helps clarify.
Cheers