These things happen (and often) and although it's a drag, it usually makes for a more well-rounded and better experience overall. Still, it's always a kick in the pants when you have to delay satisfaction, especially for a game as hotly anticipated as this one.
A stand-alone expansion pack for Warhammer 40,000: Inquisitor – Martyr, Prophesy is a brand new action-RPG set in the Warhammer 40,000 universe, and although Inquisitor – Martyr is not required to play, the story is designed to fit both for new and for returning players, so don't sweat it if you aren't caught up on all the WH action thus far. Then again, now you have some more time to catch up, as the delay it pushing the game deeper into the summer.
Here's the official word from the horse's mouth:
In order to provide a more polished experience for our players, we came to the decision to set a new release date for Warhammer 40,000: Inquisitor - Prophecy and Patch 2.0. We are working hard to make Warhammer 40,000: Inquisitor - Martyr's big 2.0 update and the upcoming stand-alone expansion as great as possible and ask for a little bit more patience from the players.
The next patch (Patch 2.0) will hit Inquisitor - Martyr on June 20, 2019, while Inquisitor - Prophecy will come out a bit after that, on July 30, 2019.
In the meantime, you can find a detailed compendium of all the new features coming with the 2.0 update right here.