Can confirm this is happening, have tested with the following items (using the inventory):
Black Couch: Downward-facing, can't place pixpets or items
Striped Couch: Down, can place pixpets and items
Gothic Couch: Down, can place pixpets and items
Beige Velvet Sofa: Down, can place pixpets and items
Cyan Couch: Down, can't place pixpets or items
Aqua Couch: Down, can't place pixpets or items

Looks like it might just be the Standard Couch (and Standard Couch-based couches) that are affected. Housing decorator works as intended; can place pixpets/items on a down-facing SCouch, can't on a side/up-facing one (also can't click and rotate the couches worth a darn, but that might just be my internet).

PC Info: Win 10, Firefox 126.0.1