The world-of-goo package installs both the 32-bit and 64-bit binaries regardless of target architecture and depends on a startup script to choose the correct binary at runtime. This means that even on a 64-bit system, the 32-bit binary is present. Since this package uses libsdl, this means that the 32-bit binary wants its compatibility package, app-emulation/emul-linux-x86-sdl. This isn't a dependency, though. Thus, portage-2.2's preserve-libs keeps reinstalling world-of-goo, insisting that it's missing the emul-linux-x86-sdl package.
Incidentally, the game "osmos" has the same type of issue, and the sample ebuild (in bug #327435) resolves it by installing only the correct binary, renamed to the startup script's name. We may not want to do that here, since I believe the startup script is more complex, but it can be a guide for a solution.
Confirmed, the installer should remove WorldOfGoo.bin32 or .bin64 depending on the arch.
FYI, the patch I uploaded for bug 460626 also includes a fix for this.
Created attachment 367544 [details, diff]
Here's a slightly cleaner fix for this bug.