Some fluctuations during a print are to be expected as the print head moves and changes the reflected light.
Turning the lamp on/off, or changing resolutions 720p/1080p should force the exposure to update.
Adjusting video display settings on a PC can help with brightness/contrast, but if the dark view is present on both computer and Handy the issue is probably with the printer.
Thanks for your explanation. But I have also noticed this phenomenon on time-lapse videos. You can already see that there are differences in brightness in the preview of the recorded videos. Normally, prints with black filament are also recognisable (this is also not a PLA matte).^