but when displayed with Bing image as background, the M9 show total transparency in what we see as bright blue in the M8 version.
Tell us what settings you are using in 9, for example, in the Style panel, and tell us about the images. Saying "it is a tiff" says nothing. For example, how many channels do they have? 1 ? 4? The user manual for the software you use to create the images can probably tell you what it is creating.
Just a wild guess, but it sounds like you have a four channel tiff image where in 9 one of the channels is being used for the Alpha, that is, transparency channel in 9. 9 does a better job than 8 of extracting data from tiff files, providing them to you for your use as you see fit.
If that's it, read the manual topics on Style for images as well as the various examples and you should be able to sort it out very quickly. If that's not it, doing that reading will help you learn a bit more so you can provide more details on what the images are and what settings are being used.