Is there a missing scene in Tenet or am I missing something?
After the protagonist, Neil and Kat went to Oslo and inverted back to normal time, the Protagonist went to speak to Priya. The scene after that all three are in a ship that is going backwards, the protagonist doing pull ups wearing a mask. This means that they are inverted, but how and when did they invert?
You're right, they don't show the inversion between the Protagonist meeting Priya and the assault team on the ship.
But we later see that they have turnstiles on the ship, so they probably used those same turnstiles to invert the team so they could travel back the two weeks needed.