This drop may suggest that Skip-gram concatenation is not appropriate for this configuration.
the concatenation of CBOW and Skip-gram models (the Concat approach) still improves the results as we can see for BC (BC ∪ JRC) where we move from 53% to 56.1% of MAP score and for BC (BC ∪ CC) where we increase the MAP score from 68.4% to 70.9%.