elpepin
When you are doing your HandBrake variable to constant frame rate, use the H.264 (x264) Video codec under the Video Tab.
I typically replace "Chapters" with seconds.
Container = Mp4
Leave the Picture Tab as is.
Head for the Video Tab.
Video Codec = H.264 (x264)
Dot the Constant
Dot the Constant Quality leave at 20 RF
Uncheck web optimize
Leave Presets at Regular/Normal
Leave the Audio Tab as is.
Guideline for now is to make only the changes necessary - change a variable frame rate to a constant one.
By the way, why are you using a HandBrake Average Bitrate = 1500 kbps (kilobits per second) when the source is 12 Mbps (megabits per second)?
Hit the Start Button at the top left of the workspace, and wait for the "Queue Finished" in the progress bar at the bottom left of the workspace.
Have you done that?
Please give it a try and then let us know the results.
Thanks.
ATR