I’ve never come across this, but it seems it was a silent addition to make frameworks in OS 10.6+, see https://apple.stackexchange.com/a/221210. In my System Prefs the suggested setting is ‘off’ but the auto-complete you describe works in some parts of Tinderbox. It’s less to do with the map vs text as whether you are typing in a text input box such as the title ($Name) edit box in any view pane, or in the Inspector or other dialogs.
The main text space is a different thing and not a simple input box, which is likely why the behaviour is different - i.e. the ‘box’ doesn’t use the same underlying framework elements , or they’re configured differently. I’d suggest you email Eastgate, explaining your need.
As a side observation, I’d note that with a move towards a ‘soft’ escape key on some Mac keyboards, perhaps Opt+Esc may not remain a got shortcut for this functionality. I tend to use TextExpander for expansions as auto-expansions can be a bit hit-and-miss in their behaviour.