I don’t work with broadcast wav but I do include XML from Cubase, which is where I do my processing. I tried now to strip all Metadata from the files hoping that it would fix it but still the same result.
I think you’re definitely onto something with that it involves L, R, C and S. I tried now with another set of files that funnily enough contains files ending in all these four letters. The result is that I’m getting 6x tracks: first track is with a file ending with L, second track a file ending in S, third track with a file ending in C, fourth track with a file ending in R, and the last two tracks (5-6) containing all the other files. So it definitely seems like Wavelab is splitting up the files in belief that it’s multi-channel file sets.
I can’t post any of the wavs I’m afraid, don’t think the labels would be very happy about that. 
EDIT: I think I found it! It seems to be dependent on the settings for how Wavelab handles dual mono files. In Preferences>Audio Files, in the “Dual Mono File Identification”, I recently changed what I THINK might be the default setting in “Name Interpretation #1” from “-L” and “-R” to simply “L” and “R” when I was messing around a bit with dual mono files. I now changed these back to “-L” and “-R” and now my files are working as expected. If I now change the name of any of the files to include for example “-R”, it ends up on its own track. Note that this happens regardless of if the “Allow Opening of Dual Mono Files” box is ticked or not.
As a side note: Selecting any of the “Name Interpretation #5” or #6 or #7 crashes the application for me.