It worked fine for me when tvOS 14.5 was released. And I did it again today with tvOS 15.1.1. Not sure why your experience is different. Hold the iPhone screen-to-screen, so that the front facing camera is used for measuring. There a several in-depth procedure outlines from third party websites, including TidBITS.
For further assistance, I would suggest to contact an expert from the Apple Support staff. Contact Apple for support and service (phone numbers) or start your support request online (chat/email/callback).
What is your model iPhone? Identify your iPhone model - Apple Support