FFmpeg. CUDA decode. Past duration too large. Variable FPS
The key -drop_second_field 1 fixed this issue.  And result command looks as follow: 


  ffmpeg -y -hwaccel cuvid -c:v h264_cuvid -deint 2 -drop_second_field 1
  -i "udp://IP:PORT"  -vcodec h264_nvenc -acodec libfdk_aac -f flv rtmp://IP/test/name



What I think about this problem: at the moment in cuda decoding we can use only 2 deinterlace variants: 


-deint 1 it's bob deinterlacing,
-deint 2 it's adaptive deinterlacing.


Both of these variants are increase bitrate, because after deinterlacing each field(half of frame) displaying as frame (framerate is doubled) Bitrate is grow and if fps is variable we got timestamps issue. And when we enable drop second field then everything looks fine with bitrate and timestamps.