Lu Yuanming's Absolute Vision naturally sees everything, but he cannot find Tang Zhe'an—not because he cannot see, but because he cannot comprehend.
Absolute Vision is Absolute Vision, but it is merely vision. When Lu Yuanming had the first and second Absolute Attributes, much of what his Absolute Vision perceived remained incomprehensible. However, upon condensing the third Absolute Attribute, he could finally perceive the truth of Demonization's Realm's overall structure—it was a living digestive organ.
Similarly, Lu Yuanming's Absolute Vision could now see the existence of True Spirit, but seeing did not equal distinguishing. True Spirit was a type of information he could not comprehend, also referred to as a Brand—a highly unique combination of informational carriers. He could see it, but he could not discern it.