I have never had this in MSP11 or 12, or a previous install of Pro 13.
I reinstalled Pro 13 after some problems with the PC (had to restore an image backup which preceeded the Pro 13 installation) and now I get a very strange editing problem.
I press S to split the tracks here
and when I click on the two track sections here to select them (first click on the video and then hold down Shift and click on the sound track) and press Delete
instead of deleting the selected bits, it seems to delete a load of other stuff to the right... in fact it deletes most of the video track.
It is slightly as if it was deleting everything which is NOT selected.