The Hebrew words for “navel” ( shorer ) and round ( sahar ) are each used only in Song of Songs 7:2a.  That is no doubt partly for poetic reasons, since the word faintly alliterate, and both alliterate with the verb “lack” ( chasar ) in 7:2b.

Possibly there is another dimension to this.  The closest equivalent to sahar (round) is sohar (a prison, a roundhouse, used only in Genesis 39-40).  The round “navel” of the beloved imprisons the lover, but it is a delightful imprisonment in which he lacks nothing.