Trim paths > offset

Hi 

I'm facing an issue similar to what is visible in this video - https://youtu.be/a8JUY7CZ6ZI?si=sXcBMLhTIq696jze&t=170. If you'd notice when he applies offset at 02:48, the animation stops at the anchor point of the square and doesn't complete the shape, like it normally would. Any leads/ideas please how can this be fixed? I'm really new and this is time sensitive pls.