I'm guessing the issue is with Client Authentication with certificates.
It's not something that's used very often, and it seems browsers don't handle this very well.
Perhaps this would be confirmed with Wireshark/tcpdump.
In any case, it worked for me from the Brave browser on the Mac, simply double-clicking the .p12 file and importing the certificate.
When I went to the site, it worked first time.
I also tried Chrome on Windows, but got an error that I was not authorized.
I ended up having to delete and reimport the certificate from the "Manage User Certificates" program in Windows 10.
Had to make sure "Client Authentication" was enabled.
I also had to restart Chrome, but then I was prompted to choose which certificate to provide for authentication when visiting the site.