Because it has been more than 2 months since I observed the problem and I reported it via Apple feedback, but it is still unsolved, I decided to contact AppleCare.
They were very helpful and friendly, and they finally identified the problem:
The flickering appears only when the subtitles are on and it happens every time the subtitles appear on the screen. If you disable the subtitles, or if you watch a scene which has no subtitles, there is no flickering.
They told me that they are working on this and it will be fixed in a future update of macOS. They were not able to specify if it will be fixed in the next update or afterwards, but it will be fixed after all.
Fingers crossed. 🙂