Which camera are you using? If it is a 2.8mm fixed, and maybe even a 4mm fixed, then it will be problematic at that height and distance at night.
If the cameras are positioned in close to ideal settings and you are not pushing the limits of IDENTIFY purposes, the IVS works very well at nighttime.
But you should only use one or the other. There are 3 motion detection options. Motion Detection, Smart Motion Detection, and IVS. If you are using more than one, coupled with high frame rates, that can cause an issue.
Keep in mind that these type of cameras, although are spec'd and capable of these various parameters, real world testing shows if you try to run these cameras at 30fps and high bitrates that you will max out the CPU in the camera and then the camera bugs out just long enough that you miss something. My car is rated for 6,000RPM redline, but I am not gonna run it in 3rd gear on the highway at 6,000RPM...same with these types of cameras - gotta keep them under rated capacity. Some may do better than others, but you are running the CPU higher. Surprised you haven't ran into issues, or maybe you have but unaware...
Look at all the threads where people came here with a jitter in the video or IVS missing motion and they were running 30FPS and when people tell them to drop the FPS and they dropped the FPS to 15FPS the camera became stable. YMMV
Post a screenshot showing your field of view and IVS lines and we might be able to help you tweak them or tell you if what you are trying is problematic.