You think 懂得+verb is more common than 懂+noun(or doing).not totally the same.
I think the original sentence could be 不懂得(怎麼對你)浪漫（一點）
It's about an "act" (don't know how to ....) , rather than know a fact.
If you say 不懂浪漫, it means don't know what romance is, and certainly don't know how to be romantic.
Very tiny difference though