It could be a bug, but here are some possible reasons that this could legitimately be the case:
It's not allowed inside the tent
For example, at the time I write this,
- cats are the only animals allowed inside the tent
- lamps aren't allowed inside the tent
It's only allowed in certain locations inside the tent
Some interior items are meant to be placed against walls. For example, windows. Kitchens are that way too since they have pipes and shelves that would look weird floating in mid-air. To place such items, drag and drop them near a wall. The grid will only show up where they can be placed so trying to put them in the middle of a room won't work, but they should "snap" to the wall if close enough.
Try placing your items near a wall, properly rotated so they'd "make sense" if adjacent to that particular wall (if you can't pre-rotate them, try against both the northeast and northwest walls) and see if that works.