Based on the reported behavior, this is most likely being written by the Huawei camera software, not by Google Photos. The consistent pattern found was that the extra metadata appears on front-camera images taken in Portrait mode. That points to a Huawei/EMUI camera-app metadata quirk or bug rather than anything related to file size, time, orientation, Wi‑Fi/mobile data, or the upload process.

There’s no clear evidence from the available answers that "fbt" has a meaningful user-facing purpose. It may be an internal tag or a software glitch tied to portrait/selfie processing. If the photos otherwise look normal, it’s mainly a metadata issue.

If you want to avoid it, try shooting selfies without Portrait mode, or remove/edit the metadata afterward on your computer. If it only appears after transfer, compare the original file on the phone versus the downloaded/exported file to confirm where it was added, but the current evidence points to the phone’s camera software.