This is an excellent point. The aim of the masks is for users to be aware of the past performance of the system in each catchment, with reference to climatology. By climatology, we mean a forecast ensemble based on past model simulations. Here, the masks are produced based on an evaluation criterion which assesses simultaneously the reliability of the forecasts, their confidence, as well as their uncertainty (CRPS).
Based on this, if, for a given month of the year and lead time, parts of the Jucar River Basin are masked, we can indeed not guarantee that the produced forecasts are reliable, because historically, climatology has performed better.
Please note that masks differ for each month of the year and each lead time. Therefore, even if parts of the Jucar may be masked in July, they may not be masked in August.
Note also that streamflow forecasts can perform well even when meteorological variables have no skill, due to the slow catchment processes.