You need to stop watering that thing so much lol! That temp is fine for Phallies too. That leaf you see dying is actually just a finished leaf-it is of no more use as a functioning, sugar producing part of the plant. It will just drop off if you give it a light tug. No problem.
BUT it looks as if you need to give that thing a repot! The medium is breaking down for starters, and it looks far too wet. Just let it dry for a few days between watering, dont give it water if it is already moist, and always make sure it is NOT SITTING IN WATER, that has to be one of the biggest and most common mistake. Give it a brightly lit position too,
After you gave let it dry for a few days (which it needs lol), take it out of the current pot (throw this away) and just soak it in a tub or sink of water with one or two GRAINS (not a teaspoon!) of Condies Crystals (Potash), to kill any potential fungus spores or bacteria that could harm the plants roots. Let it soak for about 20 mins, and pull off the bark from its roots, and chop pff any dead, shrivelled or mushy roots. Then, lay it out on a teatowel or a HandyTowel to dry. The leftover roots should all be green. When they develop a white waxy coating (about 20mins later), pot into a pot just larger than the plant, and put a bit of bark (buy specially marked bags of Orchid Bark from nurseries or online etc) on and lightly bang the pot to settle it into the bottom and around the roots. Repeat a few times until the pot is full.
A good watering criteria to follow with Phallies, is water whenever the top of the medium is dry. Works for me. Also try adding a few pebbles to the bottom of your pot holder-make sure there is always water just below these, so it isnt touching the pot. This gives added humidity to the root area.