Strangeness with stereo channels with mono inputs

Encountered something weird yesterday. I was tracking drums and being a bit careless I created a bunch of stereo channels, but all with mono input. Now normally I wouldn’t see this as a problem, but it did something very strange with the monitoring.

Playing back what’s recorded will yield a signal 3dB lower than when the recording is running. Doing drop-ins turned into something very weird with the signal suddenly jumping up 3dB on all channels! There were some moments of total confusion until I finally figured out what the problem was.

I thought it may have to do with panning law but the settings made no difference. I also tried the same in 8.0.40 and 7.5, exactly the same results, so it’s nothing new. But…why??