I'm curious if it's common for it to randomly switch to the default irrespective of updates.

It's not. Unfortunately threading/not threading is set on a per folder basis. There is no global setting to turn it on or off. There is a default setting for new folders though. See https://support.mozilla.org/en-US/kb/message-threading-thunderbird

Would the computer rebooting (whether manually or automatically for, say, Windows updates) cause it to revert to defaults - is that what's going on?

No, computer reboot has got nothing to do with it.

I'm curious if it's common for it to randomly switch to the default irrespective of updates.

It's not. Unfortunately threading/not threading is set on a per folder basis. There is no global setting to turn it on or off. There is a default setting for new folders though. See https://support.mozilla.org/en-US/kb/message-threading-thunderbird

Would the computer rebooting (whether manually or automatically for, say, Windows updates) cause it to revert to defaults - is that what's going on?

No, computer reboot has got nothing to do with it.

Thank you christ1 - very helpful. I didn't know if others had this problem - from what you say, apparently not.

The only way I know to change the folder threading setting is by menu (View > Sort by).

Unless there's some setting icon for that that I'm inadvertently hitting, there must be a corruption in my installation. If so, that's the only glitch I'm aware of, and it doesn't happen very often, so I will just change the setting back whenever it occurs. I might re-install T-bird at some point in the future, which should fix it if it is a corruption.

Thanks again!

I might re-install T-bird at some point in the future, which should fix it if it is a corruption.

Re-installing wouldn't change a thing. All your data such as messages, passwords and user preferences, and changes made while you use Thunderbird, are stored in a special folder called a profile. Your profile folder is stored in a separate place from the Thunderbird program, so that if something ever goes wrong with Thunderbird your information will still be there. It also means that you can uninstall Thunderbird and reinstall without losing your settings and data. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data

I might re-install T-bird at some point in the future, which should fix it if it is a corruption.

Re-installing wouldn't change a thing. All your data such as messages, passwords and user preferences, and changes made while you use Thunderbird, are stored in a special folder called a profile. Your profile folder is stored in a separate place from the Thunderbird program, so that if something ever goes wrong with Thunderbird your information will still be there. It also means that you can uninstall Thunderbird and reinstall without losing your settings and data. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data

I get that. I'm not a software guy, but I figure that there might be a corruption in my currently installed program that results in a periodic changing of the thread/not-thread setting in my profile. I can't think of another way the setting changes since I know I haven't been into the View settings other than to change it back to "unthreaded".

If updates only change a *portion* of the program (without disturbing my profile settings) then there is a chance my theory is correct (i.e., a part of the program that doesn't get replaced by the update has the corruption). However, if updates replace the *complete* program software (without disturbing my profile settings) then I would agree that my theory of there being a corruption in my program is incorrect - but if that's the case, how is the setting getting changed? 🤷‍♂️

I'll keep an eye on it and see if I can make sense of this.

Thanks again. 👍