Actually they are doing their job just fine.
If you are not shifting between different framerate and just started to play in 30fps your eyes should have no issue with it.
Well even if you are shifting your eyes should adapt in few minutes from 60fps to 30fps or vice versa.
I wonder if you have some eyes issues if it is not adapting to 30fps

The common is your eyes to works like I described.
And yes if you show a different framerate in video in intervals of 10 minutes or less for me they will be all the same and I definitivally can't tell what is in each video.
If you show them in sequence I can tell which one is higher or lower.