solo switches to manual when it loses GPS. It cannot hold position because it does not know where it is.
It also cannot return home, so it basically stabilizes in manual. Meaning it will just hover but move with wind, momentum or what ever forces might move it.
Depending on the canyon you are in, GPS could have been bouncing around, making a signal present but hard to lock position.
Many people feel solo is not responding or flying away because they do not understand what is happening, and how to maintain control in manual.
There is only one cure. Practice in manual a lot. If you have no practice. Buy a cheap non GPS equipped drone to practice with. When you are ready to try solo, take the gimbal off if you can. Move to a very large open area. And fly it in manual until you are comfortable