If I select several mono tracks and apply the reverb effect, content gets moved from one track to another and half of them get blanked out. Selecting each individually and applying reverb works fine.
That’s a bug which has now been fixed. The current version of Audacity is 2.4.1 and is available via the Audacity website: https://www.audacityteam.org/download/windows/
Much better now, thanks!