PCM5102A: analog output level not what I expected?
To be able to measure the full-scale output, you have to input a full-scale sine wave.

I do not know what signal generator app you used, but https://www.audiocheck.net/audiofrequencysignalgenerator_sinetone.php certainly is buggy and does not generate a 0 dB FS waveform.

As far as I can see, https://www.wavtones.com/functiongenerator.php works correctly. (Even a 16-bit file will bring you to within 0.01 % of the limits.)