What you are seeing is flow — the state where your child feels their best and is fully engaged. Your child is absorbed in a book and the required activity is waiting. At this age, being lost in a story is a neurological win. The brain is building language, comprehension, imagination, and sustained attention all at once. Interrupting it abruptly teaches the brain that absorption ends in disruption, which over time makes deep engagement harder to find. The goal is not to stop the reading. It is to transition out of it in a way that honors what was happening and makes coming back feel possible.
You are right in the middle of something good. Let's find a stopping spot together and then we will come back to it.