Maybe something on the page will help you.


https://www.facebook.com/help/community/question/?id=10203476569149311

Snip:

Don't know if this will work for everyone but I did the following and it brought all my groups back.

Selected my name from the left column. Then clicked "More" from the choices and selected "Groups". From there I selected "Manage" (the edit pen). It gave me an option to "View All Groups". I selected this and my groups reappeared on the left hand side.

I also had googled it, and found the same page you linked to, but didn't have patience to look at all the questions and answers, esp. when I saw that some of them were posted a year ago.
but with the snip that you gave, I did follow that, and found my groups!
hallelulah
but they didn't appear on the left-had side, so I followed another tip, and put them into a "facebook Group" which hopefully will work.
anyway, at least they are still there.

I do thank you.

(that is what I get for using the master's tools!)

I hope they'll stay put now.