Lightroom Classic v8.3 opened old catalog after update

I think you opened an old catalog after the upgrade. From 8.2.1 to 8.3 there is no catalog migration.

Roelof is correct. It would take a Lightroom 6.x or CC 2015.x or older catalog to prompt a "migration". My guess is that after the update you opened your old catalog from before Lightroom Classic 7.x  

I didn't open the wrong catalog, Lightroom did!  My lightroom preference was "When starting up:  open the most recent catalog."  It opened one that was not the most recent (either in terms of timestamp or contents).  So how does Lightroom decide which is the most recent catalog?

The following may be relevant:

Lightroom was just one of many programs that I re-installed after upgrading my PC from Windows 7 to Windows 10.  The upgrade was done in the cleanest way possible, by doing a fresh install of the OS.  This wiped out all programs but preserved my user data.  For example, the lrcat files in Pictures\Lightroom are identical to those on my external backup drive (I backed up all user data immediately before the upgrade).

So if Lightroom is using something in the program registry, that would have been wiped out.  The "bug" is that, in the given circumstances, Lightroom  should have prompted me to specify a catalog to open, but it did not.  It simply opened the wrong one.

Lightroom's default behavior (in the OS Update/Reinstall scenario provided) is to first check the default location.

• If a catalog is found it will launch.
• If a catalog is found but not current database schema, attempt to update it after prompting you that it is going to update and will update and open. 
• If a catalog is not found, it will bring you a list of Most-Recently-Used catalogs and ask you to choose or to navigate to your correct catalog. 

What you seem to describing is that an older Lightroom catalog resided in the default location and Lightroom opened it as per the second bullet above. 

If for any reason your most recently used catalog is renamed, moved, missing, or otherwise not where it was before, it will fall off the MRU list, and the three bullets above will then be followed again. 

Yeah, depending on all the circumstances, it probably would have been nice if it prompted you which catalog you wanted to open if it saw more than one.

That said, there was no need to go back to 8.2.1.  You can just reinstall 8.3 and open the "good" catalog.

While you're at it, you may want to do some housekeeping so you don't have those 2 year old catalogs from prior versions sitting around taking up space and confusing the issue.

I have a similar problem without any reinstallation - there is some sort of bug involved:

I have two catalogs in my catalog folder (master and master-2), because years ago LR updated it and I never moved master, because there never were problems. After the update to 8.3 LR suddenly opened the old one and not the recent one anymore. I noticed, something was wrong, and tried resolving the issue by moving files around and so on - of course I did not notice the different catalog name. Then I closed and reopened and suddenly everything was in order again. Today the same thing happened. I close and reopen and the recent catalogue is opened.

I checked my catalog-backups and one from yesterday is called master, all others master-2. So LR is messing things up.
The weird thing is, that if I try to open master instead of master-2 intentionally LR asks me to update the catalgue. That dialog was never there, when LR opened master unintentionally.

What this company does, since it moved to the cloud is horrible. Before cloud every update worked, now every other update involves grave problems. Adobe doesn't pay me to find and report their bugs, so it's a waste of my time again, to take care of this rubbish. Can't wait till some serious competitor comes along and Adobe is forced to allocate some resources to QC.