I don't think this is unreasonable. although there publicity surrounding the release, it was still an album of Christmas music. although it probably didn't help the quarterly statement, I expect a&m has resoundedly recouped their investment.From its release October 1978 to January 1981 it sold Gold (500,000 copies).
That is, Christmas Portrait LP did not tear-up the charts initially.
As per Billboard Charts, in January 1979, it was at #145 after five weeks on chart.