@Asherathe thanks. I am on a LTS distro so Pango 1.44.7 and Harfbuzz 2.6.4 which seems extraordinarily old, unless the versioning changed.
So will park this until I am forced to upgrade later this year when the LTS ends.
The Harfbuzz dev team seems very active. Looks like 2.6.4 was the last release of 2019. They've done a lot of work since then, and went through several versions in 2022.