mixed up cron job stderr outputs for simultaneous jobs when sent over postfix

Use case: two (non-root) cron jobs trigger simultaneously (i.e. two different crontab entries scheduled at the same time)

Context: Trusty 14.04.4 Desktop LTS
$ uname -r /// 3.16.0-50-generic
Cron version: 3.0pl1-124ubuntu2 (trusty)
Postfix version: 2.11.0-1 (trusty)

# cron entry #1
# `rsync' Mozilla FF's profile from RAM-DISK to SSD every 4 hrs.
# (FF's profile lives in RAM right from boot on.)
0 */4 * * * . $HOME/.profile; /opt/bin/fxram-sync profile

# cron entry #2
# Download an embedded picture from a specific web page every 2 hrs.
# Save with date-stamp.
0 */2 * * * . $HOME/.profile; /usr/bin/wget -O $HOME/Press-rev/Pix/$(/bin/date +%Y%m%D%H%M%S).png -o $HOME/Downloads/Logs/wget.log -- https://some.domain.net/news/that-pix.png

Cron jobs and pertinent script(s) function perfectly well.
Postfix is configured and has be stable in last 18 months.

As soon as the 2 cron entries are enabled (to trigger simultaneously) however, Cron Daemon starts sending mixed up stderr outputs (via Postfix). See example below:

===============================
    From: Cron Daemon <---@ ---->
    Subject: Cron . $HOME/.profile; /usr/bin/wget -O $HOME/Press-rev/Pix/$(/bin/date +%Y%m%D%H%M%S).png -- https://some.domain.net/news/that-pix.png
    To: <email address hidden>

    rsync: stat "/home/user/.mozilla/firefox/main/.cookies.sqlite-wal.YA5Dzv" failed: No such file or directory (2)
    rsync: rename "/home/user/.mozilla/firefox/main/.cookies.sqlite-wal.YA5Dzv" -> "cookies.sqlite-wal": No such file or directory (2)
    rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1183) [sender=3.1.0]
===============================

In the example above one gets the header subject line from cron entry #2, which otherwise produces no error, and in the message body the stderr of cron entry #1 about .cookies.sqlite-wal not being found (normal in my setup).

By delaying either one of the two cron entries by a few minutes, the email snafu disappears completely and no stderr output report is mailed.

Is this a bug ?