Reliable audio level meter/algorithm

I thought the problem of perceived vs. actual audio levels (i.e. LUFS vs. dB) had been resolved - but not so.

I realised this after recently starting a thread (“Manky Meters?”) because I thought a Project of mine had become corrupted and was giving unexpected audio levels (i.e. 48 dB high-pass filter cuts at 500 Hz produces large INCREASES in dB values).