The emerging leaves are pale, telling me that it's not getting the bright indirect sunlight it needs. The soil mix looks fine, very similar to what I use, but it's not getting watered enough. These plants love water, they just don't like wet feet. If that makes any sense. With the soil mix that you're using, you can water it much more often. Mine stays outside for most of the year and I water it almost daily. Plus, I rarely fertilize it and it still grows like a weed. That's because it gets all of the bright, indirect sunlight that it wants. It doesn't matter how much fertilizer you "feed" a plant, those fertilizers aren't food, and they won't do any good if a plant isn't getting the sunlight it needs to photosynthesize. Plants manufacture their own food, and they can only do that through photosynthesis. As for the aerial roots, you won't really see many of those until the stems grow much longer. Just give your plant much brighter indirect sunlight and more frequent waterings, and it will take care of the rest itself. These plants are called monsters for a reason, HaHa... Also, don't bother misting. Monsteras love humidity, but will grow perfectly fine in the interiors of our homes. My place uses gas heat, which pretty much strips any moisture out of the air during the winter months, and it doesn't phase my monstera one bit.

We'd love to help you out but honestly, there is just not enough information provided to make anything more than some broad assumptions or wild guesses. Were all the affected plants planted at the same time? Have you ever had a soil test done? Have you dug up any of the plants and examined the roots? How are the plants cared for.........fertilizing? mulching? watering? Are you close to any neighbors who might use pesticides? I would expect this to be a below ground issue :-) Meaning that something in the soil (or missing from the soil) is causing the problems. Might be insects or other soil organisms; might be nutrient issues; might be applied chemicals that have percolated through the soil to contaminate your plantings. Does the UofG have a plant pathology lab to do a proper diagnosis and evaluation of the affected plants?

Here"s a photo of one of my five shrubs. They all have stunted new leaves. This isn"t herbicide damage. I used herbicide three months before the problem started. I used it again two weeks ago after all shrubs already had stunted new leaves.

I'm not used to 115 degree temps but it sounds like it could be watering too frequently. Bougainvaillea likes to dry out a little between waterings. Dig down a few inches before watering and see if it's still damp. Water soluble fertilizer or granular?