Strange behaviour in colour correction filter

What is your operating system?
Operating System: Ubuntu Studio 20.10
KDE Plasma Version: 5.19.5
KDE Frameworks Version: 5.74.0
Qt Version: 5.14.2
Kernel Version: 5.8.0-53-lowlatency
OS Type: 64-bit
Processors: 4 × Intel® Core™ i5-7400 CPU @ 3.00GHz
Memory: 15.6 GiB of RAM
Graphics Processor: GeForce GTX 1060 3GB/PCIe/SSE2

What is your Shotcut version (see Help > About Shotcut)? Is it 32-bit?
Shotcut version 21.05.01

Can you repeat the problem? If so, what are the steps?
(Please be specific and use the names as seen in Shotcut, preferably English. Include a screenshot or screen recording if you can. Also, you can attach logs from either View > Application Log or right-click a job and choose View Log.)

I adjust keyframes in the color correction filter.
I set shadows to a negative value (RGB -3.7%).
I created another subsequent keyframe that I deleted.
Then this behavior occurred.
When the value reaches RGB value -0.3 % there is a movement in the black point of the graphic color circle, which is accentuated when going from negative RGB value to positive RGB value.

I am only changing the values of shadows, mids, and gain from the brightness levels (I don’t adjust color on the color circles).
So the RGB values do not change simultaneously and in some cases are slight “out of sync”.

To change these values for shadows, mid-tones (gamma) and highlights (gain), I use the mouse wheel over the greyscales, to avoid changing the colour.

I ran the check-in Windows and found that this happens with projects where I incorporate PNG 8 bit image sequences (1 image per frame).
Maybe RGB( PNG) to YUV or something similar can affect?
how to avoid this? maybe export to lossless format?

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