I'm sorry the "stable" in opera-stable should mean "it should be tested carefully and it shouldn't break anything that works?". Either way, I did the method that copied libffmpeg.so before and it worked fine until the latest update. After checking this topic, I found out Opera-stable has its libffmpeg version

dpkg -L opera-stable | grep ffmpeg /usr/lib/x86_64-linux-gnu/opera/libffmpeg.so /usr/lib/x86_64-linux-gnu/opera/resources/ffmpeg_preload_config.json

I tried remove the /usr/lib/x86_64-linux-gnu/opera/lib_extra/libffmpeg.so and videos play fine now on my PC.