Problem setting 'fileformat'

Some of the log files I collect contain both CR and CR-LF line
endings. Normally on a Linux system, Vim opens those files as
'fileformat' "unix", which puts ^M at the end of lines ending with
CR-LF. I would like to hide those ^M by forcing 'fileformat' to
"dos" for those files. I think the following should work, but it
doesn't.

au BufReadPre mary* set ffs=dos ff=dos

(I'm using a test file named mary.txt which contains a mixture of CR
and CR-LF line endings.)

Instead, whether I start vim with the file name on the command line,
e.g., "vim mary.txt", or start vim with no argument and open the
file later with ":e mary.txt", the results are the same:
'fileformat' is "unix" and ^M appear at the end of the CR-LF lines.

It can be seen from this command,

:verbose set ffs? ff?
fileformats=dos
Last set from ~/.vimrc
fileformat=unix

that the autocommand is being triggered, but that Vim is setting
'fileformat' to "unix" internally. If Vim is setting 'fileformat'
_before_ the autocommand is triggered, it is ignoring the 'ff' in
the autocommand. If Vim is setting 'fileformat' _after_ the
autocommand is triggered, it is ignoring the 'ffs' in the
autocommand. Is this a bug or my lack of understanding?

I'm running version 8.0.187 on Linux. I used a ~/.vimrc containing
only that autocommand and I moved aside my ~/.vim directory. I even
ran vim as "vim -N --noplugin", all with the same results.

Regards,
Gary

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