Why did observing a fully illuminated Venus contradict the Ptolemaic system?

Answer

The geocentric model predicted Venus could only appear as a crescent or new phase.

The Earth-centered model positioned Venus always between the Earth and the Sun, meaning only crescent or new phases should be visible; observing a fully illuminated Venus was physically impossible under that arrangement.

Why did observing a fully illuminated Venus contradict the Ptolemaic system?
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