They probably work but probably not within the same maximum/minimum ratings as the original chip. Your design might need a max frequency, or a minimum operating

They probably work but probably not within the same maximum/minimum ratings as the original chip. Your design might need a max frequency, or a minimum operating voltage, or a certain current-driving capability etc. that the fake chip cannot provide, causing the final product to fail when the design prototype worked fine. There are many variants of the 555 with wildly varying characteristics, not all are interchangeable.