Japanese House-guests from Hell (part 1)

Today is a great day. When I will be home from office our house-guests-from-hell (hopefully) will be gone for ever and never to be seen again. Actually this was the second (and officially the last time) that we - out of some emotional attachment to Japanese culture - will ever help out some Japanese friend in a kind of housing/hotel-emergency case by offering them our guest-room until their condition has resolved.
Not that they overstay their welcome in particular or that Ms. Moonshine or I would suffer from a kind of weird house-rule syndrome that would make living together in the same household for some time a kind of never-ending nightmare. On the contrary I personally think that we are probably the nicest and most relaxed hosts in this whole wide world. Since we both work, we are rarely at home. Our apartment is rather spacious to allow everyone to spread out and have some privacy when needed. We are neither overly tidy nor utterly untidy. We like to cook and don't mind cooking some additional servings as long as it does not become a regular institution. Also we both grew up in an environment where we learned to share basically everything with our brother or sister, so we do not really need to have "our" tomatoes and "their" tomatoes side-by side in the fridge.
So in summary I would say we have everything that is needed to make the stay of some guests over several days or even weeks a pleasant experience for both, them and us and so far this has been always the case...except for when the house-squatting party happens to contain some Japanese.
In such a case the outcome seems to always similar. After some time the atmosphere becomes somewhere between slightly chilly and deep-frozen and when they move out we never talk to each other again, even though we used to consider them as friends for several years.
I know this might sound like Mr. Bavaria and Ms Moonshine are just a weirdo couple that makes use of poor Japanese people's emergency situations to lure them into our dungeon and torture them until they finally manage to escape their doom, barely saving their lives. However there is this huge evidence of numerous other non-Japanese house-guests that we had a great time with together and became even closer friends and they basically always can't wait to visit us again. As a first conclusion, the reason must be hidden in the mere fact of the house-guests being Japanese...