In an interview that took place during E3 2019, lead producer Kensuke Tanabe hinted that the team created multi-floor puzzles, since the layout of a hotel would be easy to visualize. However in the final game this idea was scaled down to multi-room puzzles. When asked about this in a 2020 Kotaku interview, the game’s director Bryce Holliday said that this was done due to the difficulty of designing challenges across floors. He stated:

“Those types of floor puzzles are the hardest to develop and the most challenging to change after the fact, hence the absence of them in the final product. However, that idea of room connections is throughout the game at the room scale rather than floor scale. There are many secrets within ‘gaps’ in the map, behind oddly placed walls or rooms where the height doesn’t seem to match those around it.”
Contributed by ZpaceJ0ck0
In the hallway of the 8th floor "Paranormal Productions", there are posters that reference past titles that the game's developer Next Level Games worked on. In order there's a poster of Mario about to kick a soccer ball (Super Mario Strikers), a poster of King Boo and Luigi (Luigi's Mansion: Dark Moon), A poster of the overhead silhouetted cast of Mario Strikers Charged, and a poster of Little Mac, Doc Louis, and Mr. Sandman (Punch-Out!!). If Luigi sucks the last poster with the Poltergust, it will reveal a green boxing glove resembling that of Little Mac's.
Contributed by CLXcool