Forecast in Demand plan showing a dip in weeks though history shows consistent sales

Summary:

Forecast shows dip in the December month, though the sales history shows steady sales.

Content (please ensure you mask any confidential information):

We have a demand plan with weekly shipment history loaded for 2 years, and daily forecast buckets .

The business usually have low sales in December during Christmas period which is reflected in history as well as forecast.

However it is seen that for many products, the weeks prior to Christmas in December also shows a decline in the forecast though the history shows steady or upper trend in sales. We have tried putting causal factors to try resolve this , but that doesnt help as such. Has anyone come across similar behaviour for demand plan?