Putting outside, I meant in spring.
So, you really don't think it is transplant shock? Lack of humidity causing transpiration loss?
Now, I don't like MG potting soil, but that's neither here nor there. Remember though, that it is already fortified with fertilizer, so you won't need to fertilize for a while.
Overnight limpness could mean damping off. This pot has a drainage hole right? Do you water until you see water coming out of the bottom, then drain off the excess water? Wait to water again until top of the soil feels dry. With the larger pot than before, you probably won't need to water as frequently as before. With their thick stems, mint is pretty drought tolerant.
Try steeping a chamomile teabag and a cinnamon stick in a quart of warm water, let steep and cool overnight, and try watering with that next time it needs to be watered -- it is fungicidal and may help. If it is foliar fungal issue, then spraying with milk solution can help (1/4 milk, 3/4 filtered or de-chlorinated water).
Also, be sure to let tap water outgass the chlorine overnight or 24 hrs before using..
Something else to consider, though last night was warm for this time of the year, and mint shouldn't be affected (my mint outside is still unfrozen and i could probably harvest some of the still good looking tip growth) -- next to the window/just below the sash can be colder and frosty draft can form. On the flip side, many heating vents/mechanism are located near windows and can also play havoc with the micro-environment.
Hmmm... I can't think of anything else. What do you think?
My applemint cutting I took this fall just before first frost is in a NW window (behind a plastic sheeting) under an old aquarium light fitted with a daylight tube -- it gets no direct sun but the light is on a timer for 16 hrs. I thoroughly mist all my indoor plants first thing in the morning during the winter to make up for the heater induced low humidity. You can see the new growth and looks like it could use some fertilizer. It hasn't been fertilized since October when it was first planted.