I just had a look at ALSJ. It seems that Houston asked Charlie to take the photo of that part of the ascent stage:
119:35:53 England: Okay, and we'd like you to take pictures of the ablated paint, Charlie.
119:36:00 Duke: Okay, I'll do that. I'll do it at f/8, at about 15 feet.
119:36:11 England: Okay, we'd like f/8 at 250(th of a second exposure) and f/11 at 250 of all of the ablated surfaces.
119:36:20 Duke: Okay. (Pause) (Laughing) If I can bend back that far, Tony.
I've only done a quick skim-read of the EVA transcript up that point, but I can't find any indication that John & Charlie themselves had noticed this after they came down the ladder, so presumably it had already been noticed in lunar orbit, or maybe even during trans-lunar coast. I can't find the full transcript of that earlier portion of the mission.
After Charlie takes the photo, he has this exchange with Houston:
119:37:05 England: Okay, and when you're over at the S-band, we have a couple of more pictures of that one.
119:37:11 Duke: I just got it, Tony.
119:37:14 England: Okay, there's a particular surface on it we're interested in.
119:37:21 Duke: Stand by. Okay, just a minute.
119:37:26 Young: Let me try it through a few of these little craters here, Charlie. You know, it's hard to get to where you are from here. (Chuckles)
119:37:39 Duke: Okay, the ablated paint surfaces: there were only two, and it's the two above the ascent tank. (Pause) John, you were coming absolutely straight down when you hit. Okay, Tony, go ahead with the S-band.
119:37:54 England: Okay, we want you to take a picture of the white side of the yoke. The yoke has a black side and a white side. (Pause) And on the white side, we want at 1/250th, f/5.6, f/8, and f/11.
119:38:13 Duke: Okay, I've got the f/8 and f/11. I'll do the 5.6 one.
119:38:17 England: Good show.
And it looks the same stuff is on the corresponding panels on the other side of the ascent stage:
https://www.hq.nasa.gov/alsj/a16/AS16-113-18336HR.jpg