If that affects your internet connection it is not normal. Normally, your dhcp server will issue ips in the same range as your old ip. It is very odd that you get the 172.251.52.151 ip from the dhcp server at 192.168.100.1 or vice versa.
That’s the way the modem works. If the isp has an ip address it sends that. If the iso loses the address or drops it, then the modem switches to a private ip address.