IPv4 changes by itself

What IP are you looking at? Are you looking at the assigned IP of the network adapter? If yes, are you sure you're looking at the same network adapter? Normally when you VPN, a new (virtual) network adapter is created and the internet connection goes through this one. It is normal for it to have a different IP address.

IP addresses in the form of 192.168.X.Y are assigned by a DHCP server. As an example, your router is a DHCP server. Once an IP is assigned by the server, it has a "lease time". That means that if the device disconnects from the network for a period of time longer than the lease time, that IP address can be re-utilized within the network

It is possible that if you have a lot of devices connected to the same router, and if your computer is off for extended periods of time (usually longer than 24h) the IP that was in the past assigned to the computer will be assigned to a different device, and when the computer connects to the network again, it will get a new address.

In this case, to fix a specific address to a specific device, go to your router and look for "address pinning" somewhere around the DHCP settings.

Check for the leasetime in DHCP (LAN) section of your modem. Lowest the leasetime, your ip changes randomly