Help! iPhone 6s swollen battery!

I've just noticed that the battery in my iPhone 6s seems to have swollen. I tried to take a photo of it; it's a little hard to see, but if you look at the upper edge it's curved. It looks like the screen might pop off the phone.

I don't know when or how this happened. I just noticed it, don't think it was like this earlier today. I don't use the iPhone a lot, but I was on hold with it for over an hour earlier today (a government agency). But I've had occasional long calls before, with no problem.

I had a similar problem with my MacBook Pro a while back, and realized it was because I'd kept it plugged in all the time. The battery needed to be cycled, and once I did that aggressively a few times the swelling went away. The iPhone is not usually connected to power, only when it needs recharging. Sometimes it gets down to 20% and complains, runs slow, so then I plug it in to charge. So it's been getting cycled pretty regularly.

In Settings > Battery, it says Maximum Capacity 100%. And "Your battery is currently supporting peak performance." I connected the phone to my MacBook Pro and checked it with coconutBattery, which said it still has 100% design capacity. iMazing says "Your battery health is excellent."

Anybody have any idea what's happening here, and what I can do about it? Will discharging the battery fix it? Is there a way to prevent the phone sleeping so the battery will discharge?

Or do I need to replace the battery? Even though it seems to be in excellent health?