That one does have a hole in the stem, that was coincidental though as the others I haven't noticed any holes.
I looked at another one today and it had no hole, and the rot went all the way around. I'd like to rule out sun scald at this point.
On a bright note I picked 17 and only 5 had this issue today. Previously it was 11 out of 15 -- now at 32 picked and 18 bad. It seems the others on the vines are all OK at this point (still pretty young), with the exception of one showing some soft spot. I picked it early and tossed it.