I decided to fix this cheap unbranded scart switch. It has a function to share a signal from input B or C with the 1st input A which led to interference and
Scart-Umschaltpult »AV-100S«
Hama SCART Switch Box, 3-Way Black : Amazon.com.be: Electronics
Hama AV Selector 100S SCART RGB 3 in 1 out Switcher Retro Gaming for Sale in South Gate, CA - OfferUp