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An extensive measurement campaign of the Atacama Large Millimeter/sub-millimeter Array (ALMA) band-4 receiver has unveiled a strong frequency dependence of the cross-polarization performance, with peaks which do not meet receiver specifications. This unexpected behavior has been exhaustively investigated theoretically and by full-wave finite-element electromagnetic simulations. After ruling out fabrication and assembly defaults in several RF path components, the best explanation for the cross-polarization degradation at certain frequencies is provided by the effect of the cryostat infrared filters on the corrugated horn. The theoretical expressions derived in this analysis match all measurement results satisfactorily.