Hmm I’ve had cyphastrea grow over bubble algae but I haven’t seen it expand like that before. Maybe it grew over some other organism that is making it pop out.
I have skittle bomb on my glass. Over the course of a few months it also developed similar bubble shape. The difference, mine was on the glass and you could see it was actually an air bubble under it. We thought it was the oddest thing for air to be trapped and actually expand under the coral skin. I know we took pics of it, I'll try to find one. Not positive yours is just an air bubble but it's possible.
I assumed it was air but theres no where for it to have gotten under the skin of the coral, it's been glued to the same rock for months. Unless something burrowed up through the rock, the plug, and now is under the skin? Super weird