Forgot passcode but can get into phone through Face id

I'd advise turning off passcode and setting it up again but, unfortunately according to:

Use a passcode with your iPhone, iPad, or iPod touch - Apple Support

Your device will require your passcode when you do the following:

• Turn on or restart your device
• Press the Home button or swipe up to unlock your device (you can change this)
• Update your software
• Erase your device
• View or change passcode settings
• Install iOS or iPadOS Configuration profiles

if you have a forgotten/unknown device passcode, or have a disabled device (device unavailable or lockout screen), you have no option but to erase the device. However, you can eventually restore the data and settings from a backup you made before the device was disabled. If you end up having to use a computer and you do not have one, borrow one from a friend, or go to an Apple Retail Store or Apple Authorized Service Provider. You can also use a PC with iTunes. You will likely also need your Apple ID and password. All Apple mobile devices follow the same general procedure but the link below and other links off it provide system and model specific instructions:

If you see an iPhone or iPad unavailable message or Security Lockout screen - Apple Support