All our packages using ECM are being configured with -DKDE_INSTALL_DOCBUNDLEDIR="/usr/share/help" which works fine, only marble stands out in that its translated docs end up in the default location (/usr/share/doc/HTML).
Observed with marble 19.04.3 as well as 19.08.0, probably similar to bug 377967 (weird optional use of KF5/ECM).
marble sets KDE_INSTALL_DIRS_NO_DEPRECATED=TRUE, which means HTML_INSTALL_DIR is not being set when kdoctools_install is being run.
Fixed in kdoctools commit 0c073ecebbb1db8b6bd2fbe56ea4cff0f3bef1c3.