I have too many variables to immediately find the culprit here (hoping it's not a hardware issue). I reinstalled KDE Neon fresh (keeping just my home directory), and I'm still having these issues:

Sometimes the sleep (suspend) icon in the Application Menu is *missing*, it's just gone. I can usually still suspend by clicking my T450s power button, and then suspend is an option (similar if I choose a different leave option and then choose suspend instead of the chosen one).

Other times, after rebooting, the sleep icon is suddenly back!

This seems possible related to complete crashes that require me to force-restart the whole machine. They happen unpredictably either with suspend failing to complete or the machine won't resume later. Just maybe there's some relation to whether it suspended on its own or I set it to suspend, but not sure.

Possibly-related: I'm using an Apple Magic Trackpad 2, and that requires newer kernel. I installed the hwe lowlatency kernel from the normal repos (which now gives me 5.0). When I first boot the computer, all works. But when I resume from suspend, I always have to turn off and back on the trackpad. Prior to that cycling, clicking works, but the pressure is wrong so moving the cursor only sort of works. The cycling gets everything to be fine.

I could use advice on how to even start finding the right logs or otherwise isolating this to figure out where the failures are.

I tried with both having tlp and related stuff installed or removed. So far, that doesn't seem clearly related.

Incidentally, I was having times where the whole power management stuff was failing to load. But that was before my fresh install of updated Neon today. Although I have had some freezes (and cases where the power-button no longer brought up the leave options) since the reinstall, I haven't seen power system totally gone. But when it was gone, I saw a note in power settings that "Power Management configuration module could not be loaded. The Power Management Service appears not to be running." etc.

update: I successfully suspended, resumed, and then after resume saw the full thing where power management service is not running
trying more to investigate, I tried `journalctl -r` and I find some maybe-relevant lines:

Unknown group "power" in message bus configuration file
Unknown username "whoopsie" in message bus configuration file

acpi_call: module verification failed: signature and/or required key missing - tainting kernel
acpi_call: loading out-of-tree module taints kernel.

and deeper in history, I find some cases of "Core temperature above threshold, cpu clock throttled" but that may be unrelated

I then tried to log-out and back in, that successfully brought back the power functioning in terms of the energy settings, maybe all is mostly okay. But sleep is still missing from the main menu options (though again, sleep is available during the 30-second count-down when I choose to leave via a different option)

Update: after days of no problems (even having sleep icon back), I got a freeze and then after that several of the same problems. Power system missing, no suspend available at all until reboot, then later a failure to resume from suspended state. I still don't know how to even start investigating to isolate the issue(s). Any help greatly appreciated.

I should add this variable: I'm using a dock with dual external monitors. I notice some issues with that. For example, yesterday I removed the laptop from the dock but there was still a message in the system dock saying that suspend when lid closed was disabled due to external monitors.