u/Frequent-Issue1701

Hi, recently pulled up our bathroom floor due to minor damage. Floor was done before waterproofing regulations (it was Masonite on top of pine floorboards). There is a huge hole now, I suspect due to installing a vanity.

Do I ignore it and put cement sheet over the top or do I need to fill it it (for strength). What would you do?

One pic is the visible hole, the second is how it extends under the vanity.