A --resync is required when changing any filter settings, including --max-depth. From the docs:
If you make changes to your filters file then bisync requires a run with
--resync. This is a safety feature, which prevents existing files on the Path1 and/or Path2 side from seeming to disappear from view (since they are excluded in the new listings), which would fool bisync into seeing them as deleted (as compared to the prior run listings), and then bisync would proceed to delete them for real.