Is there a security flaw in Windows PIN vulnerable to social engineering?

A lot of us use "PIN" to sign in to our laptops. All of us like the convenience of just typing in the PIN and it auto-submits. Here is the fundamental problem. The auto-submit gives us a hint on how many digits the PIN contains because it submits exactly when you type in the required number of digits. Say, your password has 8 digits. Someone you know can come in and type 8 random digits to find the auto-submit point. Now, once I know it is an 8 digit password, I could make a guided guess that it is your date of birth with the year in 4 digits. When someone gives you the liberty to have a variable length password / PIN and you still choose it to be a length that matches the length of common things like your phone number, date of birth, car number, would it not be easy for a social engineer to hack it out? If the password has a fixed length of 6 digits, and someone has put in their phone number, it could be the beginning of your phone number, middle of it or end of it. Whereas, when you have an option to put a variable length password and you put in a 10 digit password, it is likely going to be your phone number! All I am trying to say is, that the auto-submit could give major hints on what your password could be. Also, makes me doubt that Microsoft is using a secure algorithm to store the hash. It seems like the length and the hash are linked. A secure hash would technically have the same number of characters for any input. In this case, the information about your password is probably not stored securely or there are some basic password hints that are stored onto the system itself to guide the auto-submit.