Mordekaiser Magic Penetration Bug

There happens to be a Bug with {{champion:82}} when buying the Item {{item:3135}} Since {{champion:82}} already has 25% Magic Penetration thanks to the Passive of his E he should have 65% Magic Penetration after buying the {{item:3135}} , but he doesnt. For some reason he stays at 55%. Is this a Bug or is there a hard Cap for Magic Penetration?
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