There are at least two ways to set the reset attribute: through style or through direct formatting. You may have a mixture of both. If you don’t fix both, the reset is persistent.
When working with master+sub-docs, it is of utmost importance to be consistent in formatting. The most reliable way is to use exclusively styles and forbid any form of direct formatting. To put all chances on your side, the best approach is to base all documents (master+sub-docs) on the same template. I repeat it again, but this is not enough: in a “distributed” work, avoid all direct formatting for your sanity of mind.
If the TOC screenshot represents your whole book, going for master+sub-docs is uselessly complicated and a source of problems. With present day computer power, master+sub-docs is legitimate only for reused parts across many documents (think of contracts made of library hierarchy-approved "standard" clause) or "monster" documents beyond 1000 pages.