Hi & welcome! That is a pretty tree. : )
A tree that size in a pot that small could need water every day. The species name "aquatica" tells us it won't appreciate drying out. The solid yellow of the fading leaf makes me think it is a response to being too dry. I doubt mild temps from 64-90 are causing a problem, beyond a possible increase in the amount of water it is needing in the warmer temps.
It looks like the roots will probably need a bigger pot soon. I believe it would be better to move it back to the higher light spot.
Individual leaves don't last forever. It's normal for a leaf to fall off now and then, especially after a disturbance or sudden change in environment. Some trees do it seasonally even if they are in a pot in the house, depending on the conditions. You can remove leaves manually to keep fresh new ones growing. That will also help keep it from shooting up too fast and getting lanky at the top.
I hope you actually meant F for the temp, not C. 64 C is almost 150F.
