This is not unusual, it's actually common for novel covers to have a different artstyle from the webtoon. Usually two different artists are hired, one for novel cover and if the author has more money, she would request a webtoon artist for a promo webtoon. Everyone who got catfished probably hasn't read as much promo webtoons in this site as I have.
Actually I've seen a similar artstyle to this cover,
https://page.kakao.com/home?seriesId=53913576
Must be the same artist ;;
And here's an example where the cover art is different from the webtoon (there is a translation for this promo webtoon)
https://page.kakao.com/home?seriesId=54504467
Here's the translation: https://mangadex.org/title/45784/i-became-the-black-swan-mother-of-the-white-swan-princess
And look the same artist who made another novel cover:
https://mangadex.org/title/46227/she-and-the-beast
(Hope someone translates this in english)
And another one:
https://page.kakao.com/home?seriesId=53312983
And there is another one too; this artist is working hard damn: https://page.kakao.com/home?seriesId=52429082
No one asked but the best promotional webtoon I've read with the best art is this one:
https://mangadex.org/title/46230/the-kind-older-sister-is-no-more