I am aware of the VPN issues as a result of Android 13 which is unfortunate.
However, I had gone through and edited the APN settings to be ipv4 only as suggested and it fixed it.. partially.
It seems to work soemtimes, then break, then randomly work, then break.
It always works flawlessly on WiFi though.

I'm curious if there is any explanation or fix to this?
US Tmobile, Pixel 6 Pro Unlocked

I had to set it to IPV4 and IPV6. Mine is still working with no problems

    I had swapped it to that and encounter the same issues

    Name T-Mobile
    APN epc.tmobile.com or fast.t-mobile.com
    Proxy blank
    Port blank
    Username blank
    Password blank
    Server blank
    MMSC http://mms.msg.eng.t-mobile.com/mms/wapenc
    MMS Proxy blank
    MMS Port 80
    MMS Protocol WAP 2.0
    MMC 310
    MNC 260
    Authentication type Not set
    APN Type default,supl,mms
    APN Protocol IPv4/IPv6
    APN Roaming Protocol IPv4
    Bearer Unspecified

    sheesh54 are you using Google for Internet connectivity check/Attestation/PSDS Server? I'm not saying that will work but letting you know my settings. Make sure to restart after changing all that to be on the safe side. I'm no pro just letting you know how I am set up since mine works

    sheesh54 Also, I'm using LTE only and this is my carrier settings (tmobile_us-40000000138.54)

    I am not using Google for connectivity checks, also I had my device on LTE, but I changed to 5g as preferred and noticed no difference.
    Not too sure, I can understand if it just wasnt working, but I dont see why itd work sometimes then not others

      sheesh54 I am using Google for all three of the above services. That may have no bearing on your results though.

      sheesh54 The only thing left I can think of is the VPN you're using. I'm using Nord. Sorry the above didn't fix your problem. Hopefully something will come along upstream and it'll be resolved soon

      The VPN is simply Wireguard, going to my home firewall. Unless it's an issue with the Wireguard app. But I doubt it seeing i works flawless on WiFi.
      Thanks for trying, maybe someone else has an idea.

      Curiously, where is this issue being tracked upstream? Just curious about it's progress.

        sheesh54 But remember no one has had problems running their VPN over WIFI. This is a carrier issue so it is possible that it could be how the VPN is communicating with the carriers network.

          Yes I am aware of that, maybe someone else can confirm if they are having Wireguard issues

            T-Mobile custom APN settings didn't help me and neither did the latest update. I use NordVPN WireGuard, but switching to OpenVPN doesn't help either.

            T-Mobile doesn't work via IPv4 at all in my area. If I use custom APN settings and select IPv4 or IPv4/IPv6, then I don't get any data conneciton regardless of VPN Lockdown usage. Exclusive IPv6 is the only functional APN setting for me that allows for data to work...

            I wish I knew more about the issue. There is a commit for it that describes eBPF changes, but that's too advanced for me. It has something to do with how VPN Lockdown setting treats inbound traffic on IPv6 networks.

            At least in my case, I get a good stable connection once I enable Airplane mode, disable VPN Lockdown, disable Airplane mode, connect to VPN, and re-enable VPN lockdown.

            I'm curious to know whether Google is aware of this issue and is actually planning to fix it? Has Google even confirmed awareness of this problem?

            I'd like to alert T-Mobile, Google, and VPN providers about this issue, but have no idea how to get to the right level of tech support and what information to provide. It has to be something specific network engineers can work with.

            Has the latest (October 2022) update fixed this issue for anyone? It didn't do so for me and Google didn't mention anything about it in its release notes. I doubt Google is ever going to fix this problem.

            This issue is big enough for almost all my clients to want to either downgrade to Android 12 or move to a different carrier that doesn't use IPv6 or at least doesn't use IPv6 exclusively.

              8 days later