SOLVED. (hopefully)
I now believe this was occurring due to overlapping colliders and after doing some digging there's little rhyme or reason for which one gets the mouse event.
https://imgur.com/a/uTWDpDX
In this image you can see a mini game I'…
Thanks for the welcome Chris!
Each button has its own BoxCollider2D plus a script with the following code for handling the mouse click.
private void OnMouseDown(){
string str = "";
switch (gameObject.name){
…