To update a table of contents, click in the table of contents. Then go to References > Update Table. For more detailed ways of updating the table of contents, or to create a table of contents, use the Edit in Word command to open the document in your desktop version of Word (Windows or Mac).
It certainly looks like an error to me - here I am using Mac Word 365 v15.38 (slow insider) Interesting that an INDEX field shows page numbers correctly. The only workaround I could find quickly is truly horrible and may not even be feasible with a large chapter count: 1. Ensure all the referenced documents are open in Word. Use Alt-F9 or fn-Alt-F9 to show the field codes rather than the field results 3. Modify the code of the TOC field (e.g. Remove the h switch) 4. Toggle the field codes again 5.
Select and update the field code (I used fn-F9, but the 'modern' dropdown menu in the blue box might work as well) The ToC page numbering then appears to be OK. However, a further fn-F9 re-introduced the error. If I print to PDF immediately, the page numbering in the ToC appears OK when I have unchecked Word-Preferences-Print-Update fields and -Update Links. Other approaches to a workaround such as removing h, other switches, changing the ToC to use TC fields rather than Heading fields did not show promise.