Glossary link sometimes opens glossary activity instead of pop up

When clicking the link of a glossary item in text, 80% of the time it neatly opens the pop-up window (see attachment GlossaryOK). However, occasionally (have so far been unable to find any logic), clicking the same link opens the glossary activity window (see attachment GlossaryNotOK) which causes students to 'get lost' in their course as they are now in the glossary activity and no longer in e.g. the lesson activity.

Has anyone experienced this?