I'm not an Adiantum specialist, but that looks like "water stress" to me. Maidenhairs are fussy about water. They do best with perfect drainage and freqent watering with good water. (not in standing water) Your water quality and humidity look fine. Contrary to advertizing they can be tricky to grow indoors. Even, difficult to establish outdoors.

I've grown maidenhair ferns and many other ferns successfully for years. Some think that ferns need more care indoors because they seem fragile some of these are tough plants and once they are happy were they sit many only need misting and not every day some do some don't and that is dependent on how humid its surroundings is. My method is first your plant needs to sit on rocks or pebbles in a wider container under the pot so that all the leaves can benefit from the moisture rising from the bottom or group your plant with other plants that like the same lighting this helps to add humidity to the leaves. I found that they don't like to be moved so once you decide where your plant is its happiest don't move it but it can be turned for even growth. Never fertilize new ferns give it a couple of months or more to get use to its new surrounding and you can get use to its needs and when it time to fertile make the solution weak. When I started growing ferns I went to the botanical gardens and greenhouses to see how they grew and cared for ferns this is where I got a lot of great information on how to care for ferns and picked up some rare ferns and spores not found in stores but that was back in the 1970's. Your fern looks great I hope that is continues to grow healthy.

Maybe try the houseplant forum. This one is pretty dead.

That's a good idea! It's now in a terrarium that I built out of 11x14 inch acrylic panels, sealed with a synthetic epoxy (the top isn't sealed, just saran wrapped). Hopefully after a week it stabilizes and starts to condensate. This is my last resort to keeping this plant alive, so hopefully the maidenhair likes it! I've tried pebble trays, humidifiers, keeping her in the bathroom, etc... These ferns are beautiful but they'll definitely break your heart!