From Wazeopedia
If the median segment (B) has been shaped with geometry nodes, it is not straight and cannot be used for determining parallelism. Instead, draw a straight segment connected at the same in and out junctions (where in-segment (A) junctions with median (B) and median (B) junctions with out-segment (C)). Check the angles using this straight segment and delete the segment afterwards.
''Note: Be sure the junctions are unlocked, otherwise connecting a temporary segment to locked junctions will give produce a save error even if it is deleted prior to saving.''