They were filming two Avengers movies back to back, and trying to keep the secrets of both projects under wraps.
The details of Thanos’ assault on the galaxy in Avengers: Infinity War likely would have been helpful for Jeremy Renner on set, but The Russo Brothers had a ton on their plate for their dual set of blockbusters.
As such, Anthony and Joe Russo were forced to withhold some information from the cast, which is likely what happened for Renner and his first Endgame scene.