I was able to successfully reproduce this locally with cron_grass_preview_build_binaries.sh script file.
This PR changes is not correctly handled, when addon is manually compiled (SOURCE_URL is used)
GRASS Python script module gs is not available inside mkhtml or mkmarkdown Py module, because PYTHONPATH is not setted by this commands inside Html.make):
Part of compilation db.join addon log:
VERSION_NUMBER=8.6.0dev VERSION_DATE=2026 MODULE_TOPDIR=/tmp/tomas/src/main/dist.x86_64-pc-linux-gnu/
python3 /tmp/tomas/src/main/dist.x86_64-pc-linux-gnu/utils/mkmarkdown.py db.join > /tmp/tomas/.grass8/addons/db.join/docs/md/source/db.join.md
VERSION_NUMBER=8.6.0dev VERSION_DATE=2026 MODULE_TOPDIR=/tmp/tomas/src/main/dist.x86_64-pc-linux-gnu/
python3 /tmp/tomas/src/main/dist.x86_64-pc-linux-gnu/utils/mkhtml.py db.join > /tmp/tomas/.grass8/addons/db.join/docs/html/db.join.html
Main difference is that compilation script cron_grass_preview_build_binaries.sh use Make instead of CMake.