Why does the same service work on one network but need a VPN on another?

I ran into a weird issue this week. The same site opens normally on my work Wi-Fi, but on my home Wi-Fi or mobile data, it sometimes won’t connect unless I turn on a VPN.

Then, after a while, it starts working normally again with the VPN turned off.

If this were a straightforward regional restriction, I’d expect it to fail consistently. Could a particular IP range, DNS resolver, or ISP route be getting blocked temporarily?

Has anyone experienced this with a website, game, or streaming app? Did changing DNS or switching networks fix it, or did the problem eventually disappear on its own?

When only one service is affected, do you route just that app or site through the VPN, or keep the entire device connected?