Laptop Lower-Left Click Button - unusual behavior

I am using the Mint 18 Cinnamon LTS distribution on a Lenovo T530. After upgrading the kernel from 4.4.0-38 to 4.4.0-43, I noticed that the lower left touchpad button had unusual behavior assigned to it. Instead of being a left click, it appeared to behave as a middle-click or had other unexpected behavior. Clicking in the middle of a tab in Firefox would close the tab instead of selecting it. Clicking on a hyperlink in Firefox would open the link in a new tab instead of following the link. The upper left left button still had "normal" left-click behavior, as did a single tap-to-click in the touchpad. Going into the Cinnamon settings for "Mouse and Touchpad" and looking at both the "Mouse" and "Touchpad" tabs, I noticed that "Emulate middle click by clicking both left and right buttons" was "on" by default. I turned it off and "normal", or at least expected, touchpad button behavior returned. If I turn this back on, the odd lower-left touchpad button behavior returns.

Additional info:

$ egrep -i 'synap|alps|etps' /proc/bus/input/devices
N: Name="SynPS/2 Synaptics TouchPad"
P: Phys=synaptics-pt/serio0/input0

$ cat /etc/*-release
DISTRIB_ID=LinuxMint
DISTRIB_RELEASE=18
DISTRIB_CODENAME=sarah
DISTRIB_DESCRIPTION="Linux Mint 18 Sarah"
NAME="Ubuntu"
VERSION="16.04 LTS (Xenial Xerus)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 16.04 LTS"
VERSION_ID="16.04"
HOME_URL="http://www.ubuntu.com/"
SUPPORT_URL="http://help.ubuntu.com/"
BUG_REPORT_URL="http://bugs.launchpad.net/ubuntu/"
UBUNTU_CODENAME=xenial

$ cat /proc/version
Linux version 4.4.0-43-generic (buildd@lgw01-22) (gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.2) ) #63-Ubuntu SMP Wed Oct 12 13:48:03 UTC 2016