I saw this thinking Lightning should already be included in Tbird.

https://support.mozilla.org/en-US/kb/thunderbird-calendar-integration

But it seems Lightning is still an add-on and it has to be reinstalled with each update to Tbird.

Using Mozilla products is hard enough when things don't go right. I wish they would get the documentation on the web site right.

If a new user installs Thunderbird for the very first time then, yes, they will get Lightning automatically, and a prompt asking if they wish to keep it or not. It is indeed still an add-on.

For the rest of us it's business as usual. When you update Thunderbird, it will check your add-ons and try to get updates where needed. So it ought to tell you that Lightning is out of date and offer you the required version. We seem to have a bug whereby if an established user had at some time tried Thunderbird and decided to turn it off, it sometimes becomes re-enabled during an update.

In your case I would check if Lightning was installed (Tools|Add-ons). If installed and not enabled I'd try enabling it. Then I'd try removing and re-installing it.