No there are only two undergroud acessible floors. The third one wrote that you do not have the key. It is not game breaking bug, but it doesnt look good. I started over on each difficulty and ACT and each time only two acessible underground floors.
Viewing post in The Princess And The Tower (18+) comments
There are three floors and the third one is required to progress in the game. (the level with the spectral slime)
If you can send me a save game I'll analyze it.
If you are a patron you can post it on the discord bug channel.
If not, just put it up on some cloud drive (mega etc.) and send me the link here and I'll have a look