When selecting any element on the score with the mouse, it's position is often moved (unintentionally)
S4 - Minor
When selecting an element on the score, one must be very careful and hold the mouse with the other hand to avoid changing the position of the element. There should be a delay, before interpreting the mouse's position as a move command.
Seems like your mouse might be a bit defective, sending inadvertent motions - I've never experienced this on any of my devices. Try another device, or cleaning your current one with compressed air, to see if that helps. If you continue to have trouble, I recommend asking for help on the Support forum and telling people more about your system - what OS, what device - so people can suggest settings that might work better for you.