Search the web for “Apple Quicktime gamma problem”, you will find many articles relating to this. It is a bug in Quicktime. Has been for a long time, and still (as far as I know) exists.
An alternative is AVID DNxHD codecs, which work quite well. I’ve been using them to do client exchanges; visually high-quality and efficient compression.
You may also want to try the (native) PNG codec in QuickTime. It is lossless compression, and I think free from the gamma issues.
-Jeff