Can anyone shed some light on this issue?
I think it used to be broken. I’ve never heard of recording multi-track with Audacity set to mono. I think what you have now is normal which is why this condition happened on more than one computer.
I know this doesn’t help you get your tracks back, but I know recording more than two channels from a multi-track device is hard.
I also know that Audacity doesn’t easily support ASIO. So using ASIO or ASIO4All software is not a get-out-of-jail card.
Koz