I'm sure others will chime in but Michael from Spokane recently gave some tips in the blooming in January thread. See below and good luck!

This critter is in my basement, and behind it is an open window, about 1x1' wide, and beneath it is a fan. The fan runs 24/7, and the temps are in the high 40's (at the extreme lowest) to the low 60's. BUT, the air isalways moving! I've had the same troubles you've had, but then I figured out that michelias, like gardenias, need an atmosphere that is fresh and buoyant. Also, they seem to resent the byproducts of gas furnaces, which I have.

My alba is under fluorescent tubes that are on 16 hours a day, and the fan runs 24/7. The air is in motion enough that I can see the leaves move. Until I started doing this, I had the same problems you've been having. I think the fresh air component is important too. If you can crack a window open in there, I bet it'll help. I have a Michelia champaca upstairs in my kitchen nook (I'm sitting under it!) that has shed 75% of its leaves, which, though annoying, doesn't seem to bother it. It refoliates every spring. I don't have a fan in this room. I've had this tree 4 years; I'm guessing it's around 8 years old. Hopefully it will reach blooming age soon!

If your alba completely defoliates, don't give up on it! Just monitor the water so the roots don't rot, and get it outside (or back and forth between inside and outside) as early in the spring as you can...

https://www.gardenweb.com/discussions/5060539/blooming-in-january-2018?n=45

Hello. THAT IS EXACTLY what happens to all of mine EVERY STINKING YEAR!

I have spent over 75 dollars every year for the same tree after mines died of exactly what yours is doing!

I have tried every year different methods and still the same ole same ole.

I tried a plastic pot and warm conditions in a sunny window....Died

I tried a very cool room with air movement in a plastic pot..Died.

I have tried a very bright porch with cool temps, 6o's and it died in a plastic pot!

I have tried growing pot in pots, very good draining mixes, fertilizing, and even fans with breezes nothing, they all died.

SAME THING HAPPENS TO MY LEAVES..

So this year I tried something different after it started doing the very same thing up until a month ago.

I switched to a clay pit, well draining mix, warm conditions, in my green house. I sprayed for bugs, in particular mites which I can promise you are on the leaves, and spray for disease...I also use a fan not blowing at it directly but the air in the area. So far so good!!! I also use rain water, not tap for the first time. The progression of death of the leaves like that stopped immediately!

I swear between warmth, water without salt deposits, weak fertilizer, and a clay pot for porosity, and air movement, warmer temps, spray for bugs and a disease control. I got it!! Keeping my fingers crossed. I have lots of new growth coming on and the leaves stopped turning brown like that.

Could it be the salts in my water? Warmer roots? Clay pot? Lack of pests? Disease control from cooler temps/? Dry air?Not sure?

My M. alba spends its winters in an unheated sunroom where it can get into the low 40s. We've had some sunny days where the air warmed up to the mid-70s and now there's lots of new leaves and a flower bud opening - just in time for an arctic blast next week where it might hit 15 at night. It's close to 7 ft tall but I might have to drag it into the house to be safe. Robert is probably right about the air circulation because my sunroom
can get "breezy" but the michelia seems happy *knock wood :)

Yes, what these like is full light (short of burning) and low to moderate humidity, with moderate watering (much reduced in winter) - the basic problem in many situations described will be that it is outdoor plants being brought indoors, into something other than a bright greenhouse. I also wouldn't fertilize these during winter, and like nearly all kinds of plants kept in pots with saucers these last should be dumped out promptly every time they fill.