A search indicates that this issue is of interest to Calibre users and I did not find in Google a clear set of instructions, so here they are...
Windows 10 will not index contents of ePub files by default; you must install and configure a special add-in called an ifilter.
1) Download and install the ifilter available at:
https://www.zabkat.com/blog/epub-ifi...ows-search.htm
(There is also an AZW/mobi ifilter available on this page.)
2) Type "indexing options" into the W10 taskbar search box. An app by that name should be available to select.
3) From the Indexing Options screen, select Advanced.
4) In the Advanced dialog, select File Types.
5) Select 'epub' in the file types list. Confirm the file handler "SumatraPDF EPUB IFilter." In the bottom of the same dialog, select the option "Index Properties and File Contents." OK