Rise changing color of MP4 videos

I've noticed this too - since about a year or so ago. I used to have a little title element in my thumbnail that was the same colour as the screen background (white) and it gave the pleasing effect that the title element was connected to the background. Since the "change", however, my white title effect has turned cream and it looks terrible, necessitating the need to change the colour to something totally different to avoid the colour clash.

And my thumbnail, as Jose asks above, is indeed always created in Replay as part of the processed video. Basically, my behaviour hasn't changed in creating these videos - so my assumption is that something in Rise has.