Hi guys. Yes I know, and that's what I am trying to say...
But I found my issue
I had Calibre set to auto-start and hide the icon, so the 3.48 process was still running after closing the UI... Starting the 64 bit exe just 'activated' the running process to open the UI. Killing the 32 bit process and starting the 64 bit exe indeed starts the 64 bit process - and saves my sanity.
Cheers.