In 1.16.1, the following command creates the block but appears to ignore the shape qualifier (coordinates are as used in the two attached images):

/setblock 91 67 -86 prismarine_stairs[facing=south,shape=inner_right]

The stair block that appears has straight shape. I can alter it to the correct shape with a debug stick.

I have tried including the half= qualifier, but the result is the same. I have also tried other shapes, such as inner_left and outer_right.

I have verified that this /setblock command works in 1.15.2 but does not work in 1.16.1; see attached images.