> 
> Put the attached 3 files, `f1.cpp`, `f2.cpp` and `TAGS` into an empty 
> folder.
> 
> Do:
> 
> 1) find-file f1.cpp (C-x C-f)
> 
> 2) tags-query-replace ITracks JTracks
> 
> There are 4 occurrences that are changed
> 
> 3) tags-query-replace JTracks ITracks
> 
> There are 2 occurrences that are changed, and it reports “Replaced 0 
> occurrences”
> 
> It should have found the remaining 2 occurrences and the reported number 
> of 0 is wrong.
>

Thanks for your bug report.  I can indeed reproduce this with Emacs 27.2, 
but not with the development version of Emacs, so this bug has apparently 
already been fixed.  I did not find a workaround for Emacs 27.2 alas, even 
saving both buffers and manually re-running etags does not fix the bug.