It appears that Antora Assembler is derefencing the AsciiDoc attributes in the titles (and only in the titles) when creating the monolithic AsciiDoc.

Therefore, all attributes present in titles need to have their proper value in antora-playbook.yml.

Use this command to get the list of all attributes present in titles in the upstream repository (assuming there is at max one attribute per title):

grep -ire '^=.*{' modules/ | cut -d{ -f2 | cut -d} -f 1 | sort |uniq