This has been fixed in iOS/tvOS 15.

Under the Develop menu, select Experimental Features, then scroll all the way to the bottom and click on Reset All to Defaults.

There are two areas I would explore here: Is the URL provided by your lambda function single use only? i.e. could it be that the AirPlay target is not able to access that URL because it has already been 'used' on the host device? Have you inspected the data passed to the AirPlay target to determine whether it is sending the encrypted or decrypted URL? i.e. is it possible that the AirPlay target is receiving the encrypted URL and is not sure how to handle it?