Do items in your party inventory not persist between floors? Several times I noted that my inventory (items not equipped to characters) was completely wiped. If this is an element of design, please note it in the tutorial about the consequences of moving between floors.

Likely a mechanic in the game, it may be mentioned in the GDD's but I could see it when i skimmed over them, so it might have accidentally been left out. Personally I like and dislike losing items In the general inventory between the transition between floors; since dust is lost between floors, holding extra items could be seen as hording extra dust for the next floor if you happen to find a merchant - however if you could not sell items to the merchant you would be simply hording them for heroes or other things rather than dust.