My series volume continues numbering ("Chapter 21…"), but the PDF chapter pages restarted at "Chapter 1" while the table of contents was correct. (fixed in v1.6.4)
Real defect: the printed chapter label was rebuilt from the chapter's position in the volume instead of the number in your title. Since v1.6.4 the label follows the number embedded in your chapter title ("Chapter 21: …" prints Chapter 21) in every language, and PDF/HTML match the table of contents. Books numbered normally are unaffected.