UL monsters having a higher body temperature than other AUs (common hc/?) and Hotland getting abandoned shortly after the injections happened because of how unbelievably hot it was (obv this doesn’t affect Mettaton but i don’t believe the guy wants to be alone for long periods of time no matter how much they may hate how some monsters behave) the regular monster not liking being in or near Hotland because they feel like they’re melting and that’s because they are, just like how Toriel was slowly melting herself after not having contact with anything living and relieving herself of the lust trait, being in Hotland rises their temperature to how it would be if they hadn’t relived the lust trait and if it relies upon physical activity/contact to release it who’s to say a temperature change to the body wouldn’t fool it into believing it has been a long time since it was released and making more of itself (like bacteria (so it also depends on how long it has been since you relived it and and how long you wanna be in there but it’s not like it’s happening in seconds, it is faster and gradual though, just like how a frog won’t notice it’s boiling if the change in temperature is slow enough))