I don't fully understand what's happening here. I noticed that while in tablet mode, focusing the window causes it to be completely repainted every time; if you watch closely for the first few frames after alt-tabbing into the installer window, you can see the controls gradually painting in. Presumably this is something to do with tablet mode's one-window-at-a-time behavior and how that's implemented. I'm also guessing that something is slightly wonky in that procedure and it's causing this image control, which is supposed to be completely occluded on this screen, to appear as if it were higher up the z order than it actually is. The reason I think that is because, as step 4 of the STR indicates, the bug isn't typically visible until the window has been blurred and then focused again.
It's a bit weird that this image can appear on that page at all; apparently MUI is using the dialog resource for the first options page, and then overlaying it with the actual controls for the welcome/finish page using InstallOptions. I don't know why it's doing this rather than having an additional dialog resource for the welcome/finish pages, but it means that all the controls for both of those pages actually all exist at the same time. When this bug happens, the image control from the next page is peeking out from behind the bounding box of the header label control from the current page. Another thing I don't understand is why this only happens in tablet mode and not all the time, because I don't see anything on the welcome page that should hide the image.
No matter what's causing this, calling
ShowWindow(SW_HIDE) on the image control for the screens where it isn't supposed to appear should get us around it, so that's what I'll do.