Hello floresromania,
Thanks for reaching out to the Apple Support Communities! I understand from your message that you've encountered an issue with the front facing camera when shooting a video, as well as with Siri on your iPhone 8.
You've done a great job isolating the issue to only the front facing camera. Since there is more than one microphone on your iPhone, you'll want to follow the steps in this article to test all the microphones on your device: If the microphones on your iPhone, iPad, and iPod touch aren't working.
If you happen to have a case and/or screen protector on your iPhone, I recommend testing the microphones both with and without the case and/or screen protector. This will help determine if there could be any part of these items blocking a microphone on your iPhone. This helps to isolate the issue further.
If you're still having issues after you complete the tests, be sure to follow the steps under, "Get help," at the bottom of the article.
All the best.