There are some code paths in Wikibase that are only run when Math extension is enabled. Running tests with the extension enabled could spot some issues, see e.g. failure mentioned in https://phabricator.wikimedia.org/T173949#4485102
As per considerations @hashar mentioned in T200976, probably just adding Math as the dependency extension to the Quibble job might not be the best idea. So probably this is going to be stalled until T200976 and/or T199939 see some progress/resolution.