Master bus metering -3dB

[application4]

(HD 10 TDM 10.3.2 - Windows 7)

I was checking a new PPM meter and calibrating the 192's outputs.
Mono audio channel, 1khz tone, -18dB - PPM meter= 4 on the BBC scale.
In 'peak' view on the PT channel the reading is also -18dB.
That's all fine.

When I put a stereo (or mono) master fader in the chain, the 'peak' view on the master channel (and the metering / metering plugins etc) drops the signal 3dB.

The audio channel display remains the same.
The worrying part though, is the analogue audio levels going to the PPM meter remain the same as before. So the master fader's meter levels are wrong.
There is no actual drop in output level.

Has anyone else had or noticed this problem?

[DetroitT]

Pan-Law although this shouldn't be with a mono-1-output fader with a mono master fader ...Assign 1 out from your tone source (No Panner)
Also if you have a RMS volt meter you may wish to double check since your in the mode.