I recall that there was an issue in Tk in regards to the wordstart/wordend index...

phase1geo on Nov 23, 2018 | parent | context | favorite | on: TKE: Modern Tcl/Tk-based text editor

I recall that there was an issue in Tk in regards to the wordstart/wordend index adjectives (which the 'b' command uses); however, I thought that was an issue with 8.5 version of Tk. What version of Tcl/Tk are you using? If you go to the About window within TKE, it will show which version is being used.