Link: https://supuk.ch/papers/android-natt-keepalive-vpn-bypass

Technical report on Android VPN lockdown bypass via NAT-T keepalive offload.

The direct attacker value is repeated real-network identity disclosure. A destination controlled by the attacker can learn the source IP address as seen from the physical Wi-Fi network, the fact that the device remains online, and packet timing while the keepalive remains armed. Depending on the network, the source IP can imply ISP, organization, travel state, or correlation between a device expected to be behind a VPN and a non-VPN access network.

Is this the thing that GrapheneOS fixed a while ago?

No, GOS is currently working on a solution, while AOSP simply rejected and closed the issue without action.