If I recall correctly, the PCB is multilayered. If you found the problem by visually tracing tracks, it's possible that you're missing one of the inner layers.

However, if you actually continuity tested the traces themselves, the odds are better that your diagnosis could be accurate.

You mentioned something about your amp having a floating ground? In that case, grounding the RCA shrouds might merely have been solving that issue as opposed to solving a problem in the player?