Just updating this to clarify something. The issue was not compatibility. There are numerous accounts of people using Campy 11 speed cassettes with Shimano 11 speed and vice versa. I got my hands on another 11 speed Campy cassette and it worked perfectly.

The issue was worn sprockets on 17-19-21 and also on 15T.

The wear was not that obvious to the eye but apparent when compared to a new sprocket.