Well, it says that starting with systemd v197, the naming scheme changed:
Starting with v197 systemd/udev will automatically assign predictable, stable network interface names for all local Ethernet, WLAN and WWAN interfaces. This is a departure from the traditional interface naming scheme (“eth0”, “eth1”, “wlan0”, …), but should fix real problems.
Since you didn’t state what distro are you using, and at what point the change occurred, I can’t be sure what happened. It might be that you’ve updated your system, which caused systemd v197 to be installed, and your wwan interface got renamed as per new policy.
There is an explanation on how to disable it at the end of the article.
It’s peculiar indeed. Linux networking configuration is not really my forte, and I also don’t have a SIM card in my Librem 5, so I can’t investigate that for you. Maybe someone more experienced than I can jump in and explain it.