It is not true that you should never have to enter your passcode again when setting up Face ID. There many times where the Passcode will be required such as:
- The device has just been turned on or restarted.
- The device hasn’t been unlocked for more than 48 hours.
- The passcode hasn’t been used to unlock the device in the last six and a half days and Face ID hasn't unlocked the device in the last 4 hours.
- The device has received a remote lock command.
- After five unsuccessful attempts to match a face.
- After initiating power off or Emergency SOS by pressing and holding either volume button and the side button simultaneously for 2 seconds.
If the device is having trouble recognizing your Face, then turn it off and then back on to set it up again. As for security with Face ID:
The probability that a random person in the population could look at your iPhone or iPad Pro and unlock it using Face ID is less than 1 in 1,000,000 with a single enrolled appearance whether or not you're wearing a mask.
About Face ID advanced technology - Apple Support
As for guessing a passcode before the device is locked due to too many attempts, that is virtually impossible. For Face ID the probability drops when also using an "Alternate Appearance".