Dear Paul, If this thread addresses your question:
https://lists.gnu.org/archive/html/lilypond-user/2025-02/msg00189.html I think you are basing your MWE off of this, am I right? https://lilypond.org/doc/v2.25/Documentation/snippets/expressive-marks-_002d-contemporary-glissando In the referenced message thread you will find that there are people (myself included) who pointed out that it should be updated with the one using \after function. Yoshi