For your information, I released tonight the version 6.4.2 which fixes a nasty regression which detached joined walls, when split with Plan > Split wall menu item or recomputed with room modification pane.
Please, update to this new version if you're using Sweet Home 3D 6.4.

By the way, I'm happy to announce that the 28th language supported in Sweet Home 3D will be Thai. The translation was performed by กันย์ชัชชัย โพธิหล้า (Kanchatchai Potila) and its SH3L file is available in Translations page. Many thanks for his contribution.