That seems quite strange. Looks like healthy leaves that have dehydrated and died. That suggests water has suddenly been cut off from the root system - either the roots rotted or the soil around the roots is too dry. I'd expect a fungus or other pathogen would make the leaves look more unhealthy before dying. Or were they exposed to very cold air?

The stems had black spots one day then fully black and wilted by day 2 soon after the leaves dry out. Same thing happened last winter. If this happened in spring or summer I would just half it and allow it to come back. If I cut it now with not enough sun may not have a chance of recovering.

I'm not convinced that the dark spots on the stems are the cause of the leaves wilting like that. It seems there's something going on around the roots. The plant won't need much sun without leaves, it does require warmth. You can still cut it back and when/if leaves begin to shoot you could put it under grow lights.

I will this weekend.

Is that a heater it's sitting next to?

Yes but heat is not yet on. I cut it more than half and under the bark was some kind of yellow mold or fungus. The dead leaves are stuck on and won't shake off so it's Verticillium wilt. Same symptoms and ending as last season.

Got to get a handle on it because I brought in all my others this past weekend. I'm looking at them daily

I will keep you posted on it as I also believe it was the new soil. All the others are still in original FL soil but that one was put in fresh potting mix same brand from last season. I also suspect my humid sunroom didn't help. My real worry is did any spores spread. If I lose this batch I'm giving up on them until retirement and relocation.

You could try some Mango seeds as a cheap alternative to buying plants. That way you could test some different brands of potting mix before putting a bought started plant into it. I assumed commercial potting mixes had to be sterilised before being released. Obviously I assumed wrong.

I have never been successful getting mango seeds to sprout, stopped trying about 3 seasons ago. I've tried for many years before that with countless seeds. Once had seeds brought back from a Caribbean cruise long ago and none sprouted.

That might be because the fruit has been cold stored for too long and the cold has penetrated right through the seed. Imported fruit might have been irradiated. All they need is very warm, moist (not soggy wet) conditions. Some of the comments on the internet about how to germinate Mango seed make me shake my head in disbelief. Provided you have viable seed simplicity (and warmth) is the best approach.

Update:

Take a look at some of the others, they are still in FL soil and mostly doing good and pushing new growth. One is starting to die back after dropping most of the leaves so I will isolate it maybe in my cold citrus room.

I think the mistake you are making is thinking that window has enough light to make the plants metabolism happy. Its not. I would STRONGLY suggest you buy the brightest LED lights you can afford. Maybe just a plain LED driveway spotlight..30 to 100 watts of that should be enough. Or, a group of energy saver spiral fluorescents wired together. YouTube has plenty of cheap lighting options how- to for people who can grow tomatoes indoors on those lights alone.

I saw Home Depot has a very nice plant grow LED..for $99.

Right now,that low metabolism is causing you all kinds of problems..the soil is probably too wet too long..viral,fungal. Mangoes love lots of light.