iPhoneX suddenly changed to requiring a 6 digit passcode instead of 4? Why?

Hello,

I see you are asking why your device should start asking for a 6 digit passcode when it previously only required a 4 digit code. This is not part of the "related" article at the top of this page. That (and pretty much the entirety of a previous response) talks about what to do if you forgot your passcode and now need to reset your device.

The 4 and 6 digit passcodes are discussed in this article Use a passcode with your iPhone, iPad, or iPod touch - Apple Support whereby the 6 digit version is the default version but you can revert to a 4 digit version. However, presuming you did not change the settings and enter a new passcode, your device should not be reverting to a 6 digit version unless you completely omitted mentioning you reset your device prior to this. If you didn't do that then you may wish to contact Apple about this.

As an immediate solution you may simply have to reset your device as instructed in If you forgot your iPhone passcode - Apple Support but the puzzle still remains as to why it should have suddenly changed from a 4 digit to a 6 digit passcode unless there's a lot of missing preamble to your question.

To contact Apple, click here --> Choose your country or region - Official Apple Support

Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service by telephone --> Contact Apple for support and service - Apple Support

The link on top of this page which you had visited, before posting this question here, is shown under the "Related Article” that may have given you an answer. 

You need to mention which part from the above article was not clear to you.

Screen Lock Passcode: By default 6 Digit

Screen Time Passcode: By default 4 Digit

If you forgot your passcode, you can use your Apple ID and password to erase and reset your device without a computer.

If you enter the wrong passcode on your iPhone Lock Screen too many times, an alert lets you know that your iPhone is disabled. (Learn what to do if you see "[Device] unavailable" or "Security Lockout" on your iPhone or iPad.)

Lock Screen Passcode or Screen Time Passcode.

Following this process is the only way out if your iPhone is disabled. The steps given in this link will help you reset the passcode -> If you forgot the passcode for your iPhone, iPad, or iPod touch, or your ...

Note: If you have forgotten the Screen Time Passcode then restoring your device from a backup after the passcode reset will return the Passcode back to your device. So, please do not restore from a backup.

iCloud uses Apple ID credentials.

If you do not remember the Apple ID Password then follow the steps given in this Apple Article -> If you forgot your Apple ID password - Apple Support

If you do not remember the Apple ID then follow the steps given in this Apple Article -> If you forgot your Apple ID - Apple Support

If you are the owner or the owner has shared all information with you then to open Activation Lock:- If it is Activation locked, read here --> Turn off Activation Lock - Apple Support Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Learn how to remove a device from a previous owner's account.

If you are not the owner and do not have the Apple ID and Password then there is no way that you can get into this iPhone unless the original owner shares the Apple ID and password with you. If you are lucky you can get into this iPhone if they (owner) accidentally remove the device from their Apple ID.