EDIT:
Maybe it was just a temporary glitch. The issue seems fixed now. :-)

Old text below:

Hi! I noticed something strange: I searched for "Master of Orion" (the 2016 release) on GogDB. The database shows the "Revenge of Antares Race Pack" DLC as not available for purchase (italic font, in store = false, no store link). Yet, the DLC is actually available here:
https://www.gog.com/game/master_of_orion_revenge_at_antares_race_pack

On closer inspection, I notice that the DLC relationship doesn't seem to be correctly defined for this product. Instead, the store page lists a "Buy all games in the series" option.