I finally figured out the problem… sort of.
First, after restarting the computer recently, I started having the touchpad-stops-working-after-login problem even on kernel version 5.15.0-57-generic. I’m not sure why that was the case, but it got me looking for other solutions, which led me here: dual boot - Ubuntu 16.04 touchpad stops working after login - Ask Ubuntu
Even though the touchpad wasn’t working, I could still find it in the results for xinput list
as “SynPS/2 Synaptics TouchPad”. So, by using xinput --list-props 'SynPS/2 Synaptics TouchPad'
, I could see that one of the results displayed was “Synaptics Off (295): 1”. By using xinput --set-prop 'SynPS/2 Synaptics TouchPad' 'Synaptics Off' 0
, I was able to get the touchpad working again instantly. Just to make sure, I restarted into the latest kernel (5.15.0-60-generic), and once again, the touchpad stopped working after login; but once again, I was able to use xinput --set-prop 'SynPS/2 Synaptics TouchPad' 'Synaptics Off' 0
in the terminal to restore touchpad functionality. (I’ll have to put that line in my .profile to see if that fixes it every time.)
Of course, that leaves the question: what it setting that flag to “1” in the first place? I haven’t the faintest idea, and seemingly neither does the AskUbuntu community. What a mystery… (if anybody knows the answer, let us know!)