Hello @uiosade, welcome ! 
Yeah as @Cedric said, it’s a gimbal lock issue, which happens only near perfect 90° angles.
Have a look at this topic (not specific to cameras) where I had explained a little bit about this issue :
To give an explanation proper to the camera, let’s consider this :
The ArcRotateCamera has 2 angles : Alpha (Azimuth, red), and Beta (Elevation, Blue)
In this situation, the camera will always be “straigth”, aligned with horizon
Which means you are missing one degree of freedom : rotating arround view axis :
In the above GIF (Blender) you see this black axis attached to the mouse ? It’s the Up Vector.
By default it’s (0,1,0) :
And you would shift it a bit when you need the camera not aligned with horizon :
As you can see, the angle arround axis is defined by the plane made by the view vector (purple) and the up vector (yellow). If you have the up vector (almost) aligned with view vector, you create a huge instability of the camera angle. You should always try to keep your up vector perpendicular to view vector