Hi spk,
In your original post, you included a link for working with formfields. Even the 'problematic' line you referred to mentions them. Given that, and your own references to bookmarks, I concluded you were working with formfields. The dropdowns you're having issues with, though, are content controls. Content controls are entirely different from formfields and don't have a bookmark property.
For demonstrations of dropdown formfield and dropdown content control implementation, see the attached.
Note 1: There are two versions for formfields - one requiring macros (the .docm file), the other not requiring them (the .docx file). The content control version also requires a macro. The macros are already present in the files concerned. Press Alt-F11 to see the VBA code. For the formfield versions, unprotect the document then press Alt-F9 to see the field coding.
Note 2: Formfields and content controls should not be used in the same document; they are not designed to work together.
For PC macro installation & usage instructions, see:
Installing Macros
For Mac macro installation & usage instructions, see:
Word:mac - Install a Macro