any chance you can share the video of the highlights you've watched so I can hear for myself? anyway, the only time I've heard of stadiums using fake cheers was during the COVID-19 pandemic (which is understandable) but at present maybe they are also using it when there are not many people watching.
I Watch on a non-official streaming TV apk, where I can watch the highlights of every match. The match Monaco vs Red Star, and Stum vs Sporting where have a few supporters but that sound like have a full stadium.
I guess they probably did use fake cheers, after reading more into it, it seems like it is not a new occurrence and has been happening for a way before covid-19 pandemic but it usually only happens on televised broadcasts. also, I saw a thread on Reddit talking about fake crowd noise on the NBA, the thread was 9 years old.