The cursor moves to the first footer in the series of same page styles when you insert a footer. If default Page Style is mirrored the cursor will go to the first footer, even or odd, depending on whether the cursor is on an existing even or odd page. If the first page style is First Page it will be unaffected by the changes. Or possibly the first two pages are Right Page and Left Page.
If you want Page 43 to be same style as previous pages then click in it. Open the sidebar, click on Styles icon, click on Page Styles icon and double click the Default Page Style entry. Page 43 (and subsequent pages if all originally same other style) will show the new footer and page number will continue in sequence.
If you don’t want the page style to change then add page number to footer on page 43