You probably have a slide mechanism that is under the slide, like 1 or 2 beams (rams) that are driven by a rack & pinion mechanism.
The side walls of the slide are not square, they are wider at the bottom than at the top by a small amount. When you move it out, the bottom goes first, then the top follows. As the bottom moves out the floor drops since the slide room is still touching at the top edge. Known as a flush floor slide, no step up into the slide room.
When retracting, the room moves evenly along the floor edge supported by nylon blocks or strips (usually!) which are mounted along the outer edge of the main floor. When the top contacts the upper wall, the slide bottom (floor) is still a couple of inches or so from the bottom edge and as the slide mechanism continues inward, the bottom continues into the trailer causing the floor to rise just as the slide is fully closed. It is pivoting at the top contact with the trailer main wall. Simple, right!!???
Yours sounds like it is acting normally. If the room is square in the opening, and the rams are adjusted properly, the tilting will correct.
As to stepping on the floor or sitting on a sofa or dinette when retracted, NOT recommended on this type of slide!! The slide floor is only supported along the inner edge when the slide is open. Most floors of this style slide (flush floor) are only 3/4" to 1" thick, and will flex down and may even pull any short walls (like dinette) away from the slide outer wall. Ask me how I know!!! I support our 13' slide with a carpet covered block during travel or storage with slide in.