Importing keywords into Lightroom Classic as non-exported keywords

Import Keywords — Do Not Export flag not applying to existing keyword names

I'm importing a plain text keyword file via Metadata > Import Keywords (tab-indented, [bracketed names] for Include on Export = off, {curly brace} synonyms). It works fine for brand-new keyword names, but when the imported name matches a keyword that already exists elsewhere in the catalog, the bracket is ignored — after import, the existing keyword's Include on Export checkbox stays exactly as it was before, checked or not.

Is this expected behavior (import merges into an existing same-name keyword and won't touch its Include on Export flag from the file), or am I missing correct syntax/an option to force it? Is there any way to set Include on Export via the import file for a keyword name that already exists in the catalog, or is manually toggling it in the Keyword List panel after import the only route?

Lightroom Classic, imported via a plain .txt file (also tried .csv, same result). Thanks for any pointers.