I dont want to be anyoing but is there a specific reason you are using 2.79 (unless your GPU does not support newer OpenGL)? You should at least be on 2.83 LTS. If it is for learning 2.91.2 should be fine too.

as HISEROD mentioned this is very probably an issue with the origin of the mesh you are trying to fit to the curve.

Have you tried applying the mirror modifier?

thanks for the clarification!

I tried to make a question out of it.

i hope that made it clearer

Judging from what we can see in the gif this is a bug. Try restarting your machine and Blender, disable addons, test if your problem is solved with a different version of 2.8 (or 2.81, 2.82). If not write a Bug report with the proper template.