While in the concept page, you can click on "help" towards the top right of the page to pull up the "Regular Expression Syntax for Concepts" section.
It validates because the 16 digits exists as your expression asks for. However, your expression is missing some form of break at the beginning or end. So any number starting with 43336 at least 16 digits or longer is going to trigger.
You will trigger on:
4333612345678901abc or 433612345678901123
abc433612345678901 or 890433612345678901abc
So try adding a \s or \p to the expression. If it is a credit card, you'll also want to enable the luhn10 algorithm in the concept builder.