mhstore error, but only when procmail invokes

Hi:

I had a procmail recipe that ran this STDIN/STDOUT filter:

    mhstore -file - -noverbose -auto -part 1 -outfile -

It works fine in a shell session or a cronjob, but
fails when procmail runs it.

    /etc/mail $ egrep -i procmail sendmail.mc
    define(`PROCMAIL_MAILER_PATH', `/usr/bin/procmail')dnl
    FEATURE(local_procmail, `', `procmail -t -Y -a $h -d $u')dnl
    MAILER(procmail)dnl

I'm not asking for a fix. My kludge/workaround is to hand off the mhstore
to cron.

I tried saving the ENV vars for the failed case and then re-running using 'env
-i' ... with the same ENV as the failed case, such that I started the process
from my interactive shell, but it would not fail.

I'm curious if there is an obvious reason why mhstore should error out --
does procmail run with reduced rights?  I can supply more detail, but
this is not all that important.   I'm running fedora 31.

--
thanks,
Tom