Why is my upper headset bearing now pressed into the cup?

This is on my mountain bike.

After returning from a ride a month or two ago, I noticed my threadless headset had developed some play. Ever since then, I’ve had to retighten it occasionally, maybe every three weeks or so? When it loosened again yesterday, I decided to disassemble and clean up the whole thing. I have a tapered zero stack (ZS) headset, where recessed cups are pressed into the frame and then cartridge bearings are slid in (it’s supposed to be a slip fit, not an interference fit). As expected, the lower headset bearing fell right out. However, the upper bearing was firmly stuck; I ended up having to use penetrating fluid, heat, and a hammer to get it out. When I went to reassemble, the same thing happened: lower bearing slipped right in, while the upper bearing refused to enter the cup by itself. I ended up having to crank down the top cap hard in order to forcefully press fit the bearing in.

Prior to this, my headset had come apart properly every time.

• I’m guessing my inability to get a good preload was due to the bearing not being free to move around.
• What do you think caused this? How shall I fix it? My thought is to install a new upper cup.
• Could my headtube be damaged? A new frame is going to be expensive!