Possible bug with regards to negative file pattern match

[Rob Gom]

Hi all,
for me it sounds like a bug, however I'd like to confirm it upstream.
Please consider the following example:
$ export LC_ALL=C
$ grep -vFf dvb.ex.all.abc dvb.all.abc |grep Ring
RingSectionFilter50,
RingSectionFilter60,
RingSectionFilter70,
$ grep Ring dvb.ex.all.abc
RingSectionFilter80,
$ grep Ring dvb.all.abc
RingSectionFilter20,
RingSectionFilter30,
RingSectionFilter40,
RingSectionFilter50,
RingSectionFilter60,
RingSectionFilter70,
RingSectionFilter80,

I would expect output having the following lines:
RingSectionFilter20,
RingSectionFilter30,
RingSectionFilter40,
RingSectionFilter50,
RingSectionFilter60,
RingSectionFilter70,

Is syntax of my command wrong? I use Ubuntu Linux grep version 2.5.4-4build1.

Both files are sorted with gnu sort, they contain much more lines than
above. In fact, when creating files with only those lines, everything
works as expected:

$ cat >dvb.all.abc
RingSectionFilter20,
RingSectionFilter30,
RingSectionFilter40,
RingSectionFilter50,
RingSectionFilter60,
RingSectionFilter70,
RingSectionFilter80,
$ cat >dvb.ex.all.abc
RingSectionFilter80,
$ grep -vFf dvb.ex.all.abc dvb.all.abc
RingSectionFilter20,
RingSectionFilter30,
RingSectionFilter40,
RingSectionFilter50,
RingSectionFilter60,
RingSectionFilter70,

I am not sure if I can post whole files, but what else can I do to
verify/help fixing the problem?

Best regards,
Robert