@stef,
There's no built-in limitation on a single public IP address having multiple GlobalProtect sessions associated, that's actually really common to come across. The first thing that I would look at is if it's just this one router in question, or if you can actually duplicate this behavior. It's possible that the NAT type of the home router simply isn't allowing anyone else to form a tunnel to the same public IP address.
The other thing to look at, however if enabled I would hope you would be getting alerts for it, is if you're possibly hitting a DoS limit you have configured. You could have a max-concurrent limit configured so your firewall isn't allowing any additional sessions from that public IP address or something similar.
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