Yes, there is a mechanism and you get a pop-up telling you that the next release is available and you’re asked if you want to upgrade to it. You have the option of being asked for it again later, or not being asked again (if you opt for this, you won’t receive another upgrade notice as it acts on your choice)
This notice would have appeared a few days-week after the next release came out; but yeah, if you’re busy on something it’s somewhat easy to give a careless and wrong error & tell it not to appear again when you only meant ‘later I’m busy’. I’m betting you got the upgrade notice; dismissed it with the ‘don’t tell me again’ option and it obeyed your response.
The upgrade mechanism & notices are generated by the Ubuntu base, not anything Lubuntu devs add; so general guides on making changes apply (eg. this link tells users how to achieve the ‘never ask again’; to reverse that decision made (often in error), it’s just the reverse of that).
The easiest though in my opinion is plan ahead.
If you were using Lubuntu 20.10 for example; that is the 2010-October release (ie. year.month of release makes up 20.10; ie. 2020-October) and add 9 months. 10+9=19 bring back to 12 months per year or 19-12 = 7 or July). Most releases occur on the 3rd Thursday of the month so you know roughly when in the month it may occur (if you want the exact date, you’ll need to look back at the release notice but 3rd (or 4th if 5 thursdays occur that month) is usual; but the month alone is all you should need for when to release-upgrade.
Addendum: the 9 month example assumes non-LTS; it’s the same for LTS except it’s 3 years instead of months; ie. 20.04 LTS has 3 years of support; so it’s 20+3 = 2023-April EOL