I never use gpu acc.

Any chance to share a sample veg project?

If there is no easy or quick fix, you could apply the effx to the media in the Project Media tab.

here is a project file:

https://drive.google.com/open?id=0BzldgVVPxli7el9SMWtLZGxTbk0

I usually do this type of thing with Color Curves and cross fading Black Generated Media.

In this project there is still a little color at your marker. The amount of color left in the fade at a particular point is influenced by the Fade Type.

This fade was just an example for the bug (if its a bug) My main problem was dissolve between gopro and sony clips.

"I discovered that when i use levels as track fx and make a fade from around 70% of the dissolve picture content disappear, and only grey is visible."

So is it normal by everyone? Color curves do the same thing by me.

Here it is normal

hmm, is your project in 8 bit pixel format? if i set to 32bit, it's good.

hmm, is your project in 8 bit pixel format?

Yes, on laptop from signature

As they say in the trade, "Well, I'll be dipped..."

Using your levels.veg project on my old machine with an nVidia GTS450, I see the behavior when the GPU is turned Off.

I don't see the behavior when the GPU is turned On.

Using your levels.veg project on my new machine with an AMD RX480, I see the behavior when the GPU is turned Off.

I don't see the behavior when the GPU is turned On.

The behavior appears to be affected by the GPU acceleration setting, but not necessarily associated with a brand of video card.

so is this a bug?

Cornico: set to 8bit pls

This is not a bug. Vegas goes absolutely correct. It's hard to explain it through Google Translate. Fade applies to Event FX. If you want to track FX, then use the on track: Insert/Remove Envelope -> Composite Level. Try it and you will not see the difference: https://postimg.org/gallery/1dqpiklsa/

Cornico: set to 8bit pls

????????? It was and still is.

Cornico: ok, sorry

vitus: if it's ok, i can accept it. But why is different at Cornico?

The Pixel Format number of bits is enshrined in your levels.veg example project.

On my system it's same as john_dennis reported. There's a different behaviour between GPU on and off. GPU on does a smoother color fade.
Or to be more precise:
While fading, the color processing is different between GPU on and off, which you could clearly see when watching the vectorscope while stepping through the fade frame by frame and comparing GPU on/off.

There is no relation with the GPU. Simple math and sequence of actions. Using Levels in TrackFX given a new count of the coordinate system. Anything below a set value Output Start - no color components. 0.200 - about 50 in the histogram. Below that Vegas can operate only the luminance values.

There might be no relation in theory, but actually there is a difference appearing which you can see in the preview and which is pretty clearly shown in the vevtorscope, while the only variable triggered is GPU on/off.

Here also the same as Marco and John.
GPU off in Options/Preferences/Video

result

Cornico, In 10th version of Vegas where GPU acceleration yet was not - it looks similar.

If there is no easy or quick fix, you could apply the effx to the media in the Project Media tab.

Thats not a solution. What if i have 50 gopro clips?

I have to add media fx one by one? I cannot select all, and add fx :( why?

You could use a script to apply a media fx to all media (or selected).