Are you running the Studio LIve Mixer directly into your computer or through a hub?
You might also try changing the USB cable to see if there might be an issue with it. The AR
xx USB series is a USB2 interface. It's possible that a ratty cable could be limiting the bandwidth to the mixer. I doubt that's the culprit, but it's still worth a try.
I've tried to duplicate the issue with my AR12USB and haven't been able to duplicate it. I've tried it on both my main computer (eighth generation i7 running Win11) and on an older Dell i5 4th generation, with both v5 and v6 of Studio One. I saw a post in the Presonus forum regarding pretty much the same problem with a SL AR12USB, but the resolution seemed to be in downloading and reinstalling UC, which you have already done.
My Windows 10, 64-bit computer is only recognizing the first two I/Os of my StudioLive AR12 USB. ... ideal solution, but I know it works every time.
answers.presonus.com
I even tried it with Studio One v5 Prime to see if that might be the cause. Prime will only record 2 channels at a time, but it still showed all of the channels on my computers.
I'm not sure what Presonus' support policy is, but you might consider opening a ticket with them. It can't hurt. Worst they can tell you is it is discontinued product and they no longer support it.
One other thing you may have already done, but you might try completely uninstalling Universal Control and then downloading and installing the most recent version. If you do, make sure when it opens the drivers list that the only item checked (besides UC itself) is the Studio Live AR driver.
Out of curiosity, have you ever downloaded and installed Capture from the Presonus website? The reason I'm curious about it is because if Capture recognizes your mixer as a SL AR8USB, it will display all 8 inputs when you attempt to record. If it doesn't recognize the AR as a Presonus product, it will only allow 2 channels to record.