Or the adjustment/transcoding process, anyway; best to keep masters in original format.
This problem is so widespread that it allowed a relatively early online mass-media use of 60fps to create a *retro* effect combined with letterboxing: that of the music video for "Finesse" by Bruno Mars. By jumping into a 4:3 frame from 24p to 60p, it instantly transported you back to the 90s watching In Living Color on a tube television, and yet also taking advantage of a relatively new feature of YouTube.
I was among the first to use it when it became available in 2013, releasing my new videos in 720p60, the best my dSLRs captured at the time; unfortunately, after ten years it appears they've been resampled or frame-dropped to 30fps, so they'll need a re-release at some point when I'm able.