I am using Audacity 2.4.1 in Windows 10. If I have a project containing several stereo tracks, each about 45 minutes (they are from spoken word tapes). When I try to mix stereo to mono, the first track has no problem. But any other track takes at least 5 minutes longer to process, and when it is mixed, the program adds at least an hour, sometimes as much as 8 hours, of silence at the beginning of the track. It does not matter which track I mix first. The further down the project the track is, the more time is added at the beginning, and the longer the mixing takes. This did not happen with earlier versions of Audacity. Is this a bug in Audacity 2.4.1?
Congratulations, you’ve discovered a new bug. 
I think you should be able to work around by ensuring that every track has audio at the start of the track.
If any tracks have empty space at the start, select a section at the beginning of the empty space, and generate silence into it. If the track has audio (or silence, but not empty space) at the start, then mixing down to mono should work correctly (it does for me).
I’ll add this bug to the bug tracker as a “priority 1” issue so that it has to be fixed in the next release.