Although I can understand why you might associate the number substitution with Siri, she is not involved unless you are issuing verbal commands such as, "Hey Siri call, Joe Blow". The number association is a search function that works regardless of whether the information comes from the keyboard or Dication. The problem occurs in Messages, Phone, FaceTime, and Mail.
I am using macOS 12 and iOS 15 and on both systems, when I am in Messages on the To: line, if I start typing a number, Messages automatically presents a drop down list of every entry in Contacts list that matches the entered digits and displays them in a descending list. So for example if I type the number 8 every entry the has a phone number beginning with 8 is shown on the list which becomes shorter as subsequent digits are entered. The same thing happens if I start entering a name except the list includes entries that have the same first letter in the first name, last name, or email address. It works the same way whether I am typing using the keyboard or dictating verbally.
If I am using Siri and say "hey siri send joe a text", she responds "Okay which one…" and begins to read all of the Joes and each of the phone numbers and email addresses for each Joe in the contacts list and only stops when I say something like home (the home number), or that one (the last number read).
Mail, Phone, and FaceTime all work the same way as Messages and probably use the same API (Application Program Interface) to do it.
MY POINT IS: It appears Apple has recognized your problem and taken steps to remedy it. The downside is the remedy requires upgrading to iOS 15 and macOS 12.
