Serial port permission denied for non-root user with dialout group privileges

[Rob McGillivray]

lsb_release:
Description: Ubuntu 18.04 LTS
Release: 18.04

I write systemd services that launch with root privileges and de-privilege themselves to a system account (called mxdaemon). These services require access to the physical serial port (/dev/ttyS0) on the PC.

Serial port: crw-rw---- 1 root dialout 4, 64 Jun 13 22:00 /dev/ttyS0

When run as root, the service has no problem communicating via the serial port.
When the service is configured to de-privilege to mxdaemon, I get a permission denied error when attempting to access /dev/ttyS0.

mxdaemon is a member of both the dialout & tty groups: mxdaemon : mxdaemon dialout tty
Even after rebooting the PC, I still get permission denied for any user besides root.

Hours of scouring the Internet suggest that everything is configured correctly.

[Ubuntu Foundations Team Bug Bot (crichton)]

Thank you for taking the time to report this bug and helping to make Ubuntu better. It seems that your bug report is not filed about a specific source package though, rather it is just filed against Ubuntu in general. It is important that bug reports be filed about source packages so that people interested in the package can find the bugs about it. You can find some hints about determining what package your bug might be about at https://wiki.ubuntu.com/Bugs/FindRightPackage. You might also ask for help in the #ubuntu-bugs irc channel on Freenode.

To change the source package that this bug is filed about visit https://bugs.launchpad.net/ubuntu/+bug/1776805/+editstatus and add the package name in the text box next to the word Package.

[This is an automated message. I apologize if it reached you inappropriately; please just reply to this message indicating so.]