Well, my first thought is that it may be the water you are using. Can you explain what you mean by "mineral water"? Do you mean a water that is high in dissolved solids such as calcium and magnesium? Or do you just mean "spring water", which may or may not have a lot of minerals.
Miracle Fruit likes very acidic soil, and many mineral waters can have a high pH.
When the soil pH is too high for them, they can't utilize iron, so maybe try a soil acidifier like sulfur.
Also, it may be that you are overwatering. Yellow leaves and brown tips can be a sign of overwatering (or underwatering, or too much fertilizer...).
60 degrees outside shouldn't hurt it.
Because I have a mature one of these plants, I have grown a LOT of them from seed, and sometimes they just crash and don't recover, and I never know what did it.
Maybe other folks have some other ideas.
Good luck!