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Capricorn - there's no sign of an epub iFilter on my Windows 10 Pro (Upgrade).
If you tell it to index epub contents, it will default to the text iFilter, IIRC Windows has always done that. But that's no use, to index the content of an epub requires it be unpacked and the HTML, XML, and JPEG files be processed by the corresponding iFilter dlls.
If your epubs are being indexed can you provide details of what iFilter you used and where it's located. That screenshot comes from Nirsoft's SearchFilterView utility.
BR