When you drag your 1080p50 clip from Project Media to the Timeline, the program is supposed to set the project preset
to match the properties of your source media.
But, make sure it does that correctly by seeing what is set in Edit Menu/Project Settings/General.
If the program cannot interpret your video properties correctly, it will default to PAL/AVCHD/Full HD 1080i25 (PAL setup) or NTSC/AVCHD/Full HD 1080i30 (NTSC setup). If that should happen, then you start a new project and set the project preset yourself before importing the media via Add Media, that is,
a. Go to File Menu/New/Project
b. Set the project preset for 1080p50 as indicated in the above path
c. Before exiting the New Project dialog, make sure to have a check mark next to "Force Selected Project Settings on this Project".
d. Then import you 1080p50 into the project with Add Media.
Please let us know if that worked for you.