Possible error compositing using ADD

I have been experiencing a strange problem whilst compositing clips on top of each other using the 'ADD' composite.

As I understand it, 'ADD' adds the positive luminance of each pixel, so that the composite image ends up being the same, or brighter, than the original clips used.

It should be impossible to use 'ADD' and make the clips darker.
Ir the composite is one normal image over a 100% black clip, then the result will simply be identical to the original normal image.

I have been compositing two images using 'ADD' and finding the result darker than one of the originals.
It does not happen with un graded clips, it appears to be caused quite specifically by the Lift and Gamma commands
I think that this is an error, as I have tried to replicate this using new projects and new files, and I can not make it happen.

Also, the view in the monitor is unstable - sometimes it shows the 'correct' composite, and sometimes the darker one.
It became a more serious problem when I realised that my outputs were being darkened by this process, even if I could not see on screen

I have added a file showing what I added together and what I got as a result.
I also show the lift gamma gain settings that were applied to the lower clip

I'd appreciate any advice.

Thank you!

Alan