Why would an IPv4 APN work on a OnePlus One but not a OnePlus 5 with the same SIM?

I have a T-Mobile SIM that, on a OnePlus One, works perfectly fine on T-Mobile's APN in IPv4-only mode, with an IPv4 address and everything. However, when I take it out and put it on a OnePlus 5, it doesn't work with the same APN settings unless I change *IPv4* to *IPv6* (or *IPv4/IPv6*). I'm baffled at how this is possible. How is this possible? Shouldn't all phones be able to connect to the same APNs with the same settings?