Why does accessing my public IP via HTTPS on LAN show my modem’s interface?

I’m using a ZTE ZXHN H298A modem/router (ISP-provided) and noticed an odd behavior:

When I’m connected to my local network and visit https://<my-static-public-ip>, I get redirected to the modem’s web interface.

This only happens from inside the local network (LAN). When I try from outside (mobile network), it doesn’t respond, which is a relief.

443 port is not forwarded or no DMZ configured.

Why is this happening, and is it an issue? I could not find and editable NAT configuration or Remote Management on my ISP provided modem’s interface.