Ok. I was able to reproduce some stuff. There's something borked in there.
MC19 has been long installed on this Macbook. Before I started anything, I opened it up. It showed up as an expired trial. I know I'd licensed this copy before, so I'm not sure what happened there.
However... I did swap my computer out a while back (I had a memory slot go bad, so I swapped my hard drive into another, nearly-identical model of the same Macbook). I'm pretty sure I've used MC19 Mac on this since then, but I use the Windows version in my VM more often, so it is possible that was the cause. I had to re-license some other stuff. But I swear I've used the Mac version on my laptop since, but I can't really be sure.
Anyhow, I loaded (by double-clicking it) the MJR Install Key and it licensed MC19 without drama, and it now opens up fine and shows up licensed. Then, I downloaded and installed MC20. I did NOT launch it.
Then, I double-clicked the MJR file again from the Finder. It did the same thing and showed the PackageInstaller successful message and the web page.
Then I launched MC20 for the first time ever on this machine. And I got the Trial Period has expired splash. So I cannot, and never have successfully run the MC20 app on this computer.
Then, closed the dialog and tried the MJR file again. Same result, it shows the "success" message and web page, but it must have re-licensed MC19 because when I opened MC20 again, it still showed the Trial Expired splash. I can re-test in my VM in a bit to see if, had I launched MC20 at least once first before running the MJR, if I'd have gotten the trial. For this computer though, I get no trial at all of any kind. So, sorry guys, something is badly borked there.
I was able to install the license from this state in MC20 without uninstalling MC19 however. You just have to use up one of your restores. From the Trial Expired splash screen, I hit the Register License button (or whatever it is called) and put in my Registration Code. This worked. It showed the same success message, launched MC20, and it is licensed now for me.