I think your initial instinct of sunburn was actually correct. I'm looking at the leaves of your plant, and they have prominent purple tinges at the edges of them. The root tips are also deep purple. The hyper-pigmentation is a sign that the plant is receiving high light levels (and not all plants show this purple tinge, but we got lucky with yours). The only problem is that purple-tinged leaves are right at the brink of getting too MUCH light, and it's a delicate balance to strike between getting the plant enough light to bloom without it getting too much sun. Sunburn can take a little while for the full damage to become apparent, and often it first appears as a lighter, yellow spot, then progresses to the black spot you have in the center, which is basically just cells that have died due to the burn. The yellow spots don't ALWAYS turn black, but if there's been enough damage done to the sunburned area, those cells will eventually die, leaving you with the black spot in the center, which is basically necrotic tissue that is slowly spreading.
I had a plant that got sunburned, and leaves were still going bad three weeks later, so the progression of this wound is following the path of a sunburn-type injury. This type of injury is likely going to continue to spread, and will eventually overtake the leaf.
Heat could also be an issue. Do you grow this one next to a window? It looks like it's on a table by a window. If it's really close to a window, the light coming in can burn the plant even if it's only really hitting it for a short time. Is there any direct sunlight touching the leaves during the day? Phalaenopsis plants like a bright room, but not direct sun.
When you have your next sunny day, at the brightest part of the day, feel your Phalaenopsis leaves to see if they are warm to the touch. If your leaves feel warm, they are in danger of literally "cooking." I accidentally left a Phal too close to a window for a couple of hours one afternoon, and the heat cooked the edges of three leaves. Fortunately, those were easy to remove because they were just the edges, but I have other plants that I've had to remove more leaves from, and they're pretty tolerant to leaf removal. Your Phal will bounce back after you remove the damaged leaf. But I would remove the leaf sooner rather than later, because that wound WILL continue to spread, and eventually the plant will lose the leaf anyway. And the longer it has a wound on it, the more likely it is to contract a viral, bacterial, or fungal infection through the wound. You don't want your plant getting a secondary infection on top of this.
I would cut slightly below where the lowest yellow part is. Your plant will bounce back; Phals are hardy. But I would back off the light for a little while to let the plant recover. It needs to heal, so moving it to a slightly lower-light spot for a while is a good idea. Then you can slowly bring it back out to a higher-light area.
Good luck!! I know it's sad to cut off a leaf, but your plant should bounce back just fine with your tender care.