This seems to be the case for raspberry pi v2 camera as well. Yours is the most useful observations I have come across in the entire forum.
I ran an experiment setting fps to 120 for a HD video. I set gainrange and ispdigitalgainrange to 1. I set the exposure time to 1/30, 1/60 and 1/120. All of them are identical. So that means the camera is capping the exposure time to the FPS based max irrespective of what we provide.
I also ran the same thing but with 1/10th of the exposure values (1/300, 1/600 and 1/1200) I could definitely see differences in the images, even if they are underexposed
Third experiment I ran was with 90fps (non-standard FPS). I set exposure time 1/90 and 1/120 with analog and digital gains set to 1. This showed differences.
Let me know if you find a way to increase past the limits of exposure time defined in nvarguscamerasrc and hope my findings help you
@Edison_F_A how can I change the driver for raspberry pi v2 (IMX219) camera?