What is the right way to export videos with different FPS?
Generally, all of your assets should be converted to the same frame rate as your final output frame rate before you begin editing. If you do not convert them beforehand, you may have issues with timings. Things that are supposed to be one length end up being a different length. This is especially true if you export a variable frame rate file. Suddenly a 30 second file becomes 28 seconds, or 33 seconds, or some other length. The audio and the video end at different times. Things may play right in some places, but they may be wrong if they are processed and played elsewhere.

To summarize:


Convert all assets to the final frame rate before editing.
Set your project to the correct final frame rate.
Do not export a mixed or variable frame rate file.