GUI Element is not positioned right

I can’t find a way to counter this bug. What happens is that I have a frame inside another frame, relatively positioned and scaled. It’s 90 degrees rotated and positioned, anchored to the screen, in the middle left, so that I can perform some animations. What happens is that it doesn’t scale accordingly and correctly:

As you can see, instead of fixing on the left, it goes to weird positions, and also scales strangely. How it should look:

UI is not my specialization, so I need help. Thanks!!!