[c#] C# WinForms - custom button unwanted border when form unselected

I'm having a problem with a custom button I have created in c# win forms.. The button appears fine when the form is selected but as soon as I click away from the form a border appears on the button. A good example of when this happens is when the desktop is clicked but the form is still maximised so you can see its contents. An image of the problem can be seen below:

This does not happen on all buttons, only when the button has been clicked prior (only appears on one button at a time). This lead me to believe that it was something to do with the button focus cues but these are set to false. The border is set to 0 and I also have tabstop set to false.

Any suggestions?