This bug is probably as old as multi-clip, but did not affect any built-ins besides Reverb.

Quick fixed at
https://github.com/audacity/audacity/commit/210fc288638f284b52ea64c7a20eee959b78db3c

Paul,

Please give some guidance in these comments as to what needs testing after these changes.  Which effects?  Does only stereo need testing?  What might have inadvertently gone wrong as a result of this fix (e.g. clip lines in wrong place, trunc silence failing where silences are different in channels L and R).  Does VAMP need testing too after these changes?

Also please give proper steps to reproduce.

(In reply to James Crook from comment #2)

Are my given steps to reproduce not sufficient?

I explain that Reverb alone among built-in effects may show the problem, but it might also be reproducible in third-party plug-in effects such as VST.

There are two step 3's.  I think I understand the instructions now after re-reading several times.  Thanks.

(In reply to James Crook from comment #4)
>There are two step 3's.

I have fixed the steps numbering and tidied up the steps

Testing on W10 with Audacity 2.4.0 4b76af5

Following the steps an this latest alpha I see no difference btween with and without Step 3 - and I can hear no difference.

But testing on 2.3.3 I get the same clean results

Paul wrote in the steps:
>When some of the right channel is deleted, some of the reverb of the 
>clap appears in the left channel BEFORE the clap!

I just don't hear that not  with 2.4.0 or with 2.3.3

So unless I'm doing this wrong it WORKS-FOR-ME

I can't reproduce this in any version of Audacity.
I've even tested with commit 8eee265c382e9ecc8a1ff81629f98e49b89eaf43
(the commit immediately before the "fix").

If there is / was a problem, we need more precise steps to reproduce it.

Based on Steve and Peter's testing, and my comment #2 closing as WORKS FOR ME.

Can always open a new bug if the fix broke something.