Not a regression on 1.3.12. For possible reference, see the fixed bug 123 (Generating inside a clip can modify the clip length or create spurious clips)  

Left-clicking to merge the splits does not seem to create a problem in the steps to reproduce here. There is a case http://bugzilla.audacityteam.org/show_bug.cgi?id=373#c20 where (at least one of) detaching at silences which were clips in a stereo track, splitting to mono, moving clips and recombining the stereo track has somehow produced a split line in white space before a spurious clip of i5 samples plus another spurious 1-sample clip after that. Clicking on these spurious split lines or clips crashes on Windows. 

My method of creating the clips in that case was slightly different, but also produces the 1-sample spurious clip at the left edge - simplified steps:

1 Generate 30s tone.  
2 Select a region somewhere a few seconds long and CTRL + I to split it. 
3 Select another region that does not touch the split region and CTRL + I to split it.
4 Double click between the split regions and CTRL + L to silence it.
5 Select the whole track and Edit > Clip Boundaries > Detach at Silences.
6 Spurious 1-sample clip appears to left of the clips containing audio.