A short slur mid-thickness doesn't match tie mid-thickness.
There's a strange behavior where slur mid-thickness seem to be thinner when the slur is short. This doesn't happen organically or smoothly but in a sort of snappy, one-or-the-other way. Also, it is at odds with tie thickness, which seems to have a different behavior.
See this screenshot...
MuseScore file attached.
This is 2.0 beta and seems to affect all platforms.
More here: http://musescore.org/en/node/34176 , with animated gifs and additional screenshots.