Without more information on the video files your phone shoots, nobody can be sure.
You can use the (free) MediaInfo program to see the details of the file structure. Then paste a screen shot here of the "tree view" page.
A full on guess is that the phone shoots in a variable frame rate that is not normal to conventional cameras. You can try changing the setting in the phone to a more standard format. Or, you can convert the video using the (free) Handbrake program to a more editable format.
The current version 2019 is advertised to edit variable frame rate video.