`find-grep` (aka `grep-find`) not finding a line when in an upper directory

2017-12-12 17:57:06

GNU Emacs 24.5.1 (i686-pc-mingw) with a cygwin environment installed at C:/cygwin64 on a Windows 7 machine.

M-x find-grep (aka grep-find), then:

find . -type f -name '*-2017-12-12_09.22.57.log' -exec grep -nHE 'ERROR:' {} \;

finds a number of ERROR: lines in various .log files at various depths in the directory hierarchy where I am working.

One particular file with an 'ERROR:'

./PFF/PFF/PFF.bat-2017-12-12_09.22.57.log

is not found unless I change down a directory level, starting in ./PFF, or unless the parent directory name is [at least] one character longer or shorter (but see below.) e.g. the ERROR: line is found in

./PFF/PFFm/PFF.bat-2017-12-12_09.22.57.log

or in

./PFF/PFF-/PFF.bat-2017-12-12_09.22.57.log

but never in

./PFF/PFF/PFF.bat-2017-12-12_09.22.57.log

whether the original ./PFF/PFF directory is renamed or copied to a [longer-named] parent (when copied, I have both ./PFF/PFF and ./PFF/PFFx directories.)

Then it gets weird - if I have a copy of