Hey mate
I’d first check to make sure that the proximity sensor is not blocked by a case or cover on your screen. Or, even a screen protector might be causing it (highly unlikely).
This night light filter can dim the screen sometimes. Turn it off in Settings > Display > Night Light.
I’d also try rebooting the phone. Or, failing that doing a backup/reset.
Finally, if you continue to experience issues contact Nothing support https://nothing.tech/pages/contact-support