You can record Desktop > Window or Desktop > Entire Screen to successfully include the system audio from the shared tab in a Google Meet recording. Don't forget to enable "Also share system audio" before recording.
Reason:
Under the This Tab or Custom (This Tab) recording mode, though you clicked "Also share system audio" when sharing another tab to Google Meet, our extension can only record the system audio from the original tab, so our extension is unable to record the system audio from a shared tab in Google Meet.