I haven’t used this sensor yet, but was interested, so I spent some time reading posts and blog articles by people who have tested it in action.
There are a number of gotchas that affect the readings, such as the requirement for a substantial, unobstructed volume of air visible to the sensor and well separated from walls and floors, so I’m not at all surprised that the glass cover affects the readings. Do your research!
Example discussion (among others): We Put Bosch's Revolutionary Tiny PM Sensor Through Its Paces
Also, motion affects the readings. See this forum post: BMV080 Reads “Obstructed” When Unit Is Shaken — Works Fine Stationary