Why does space cause word completion when 'accept with' is return?

Autocorrect options are all the default. “Accept with” character for word completion is set to ‘return’. Why does the space character cause word completions? Using version

Word Completion and autocorrect are two separate methods of word automation. Either can be undone by Ctrl+Z

Word completion (looks like a tooltip) is accepted with Enter and rejected with Space. The word list for this is listed in Word Completion tab of Autocorrect Options and is also picked up from the current document

Word Completion image

Autocorrect is a word replacement option accepted by Space and also Enter. This was originally used for commonly mispelt words when you would want the “neccessary” replacement always, hence the name. Its function has been expanded to save time by using boilerplate text so some entries might not always be appropriate. The words for this are listed in Replace tab of Autocorrect Options so if some are causing problems you can easily remove them or change the trigger combo that inserts them. You can add Exceptions in that tab, say for particular abbreviations, etc. The Options tab allows you to ignore some other potential corrections.