Custom toolbar icons not truly transparent?

It seems like with custom icons, CorelDraw just fills transparent pixels with the UI's color? When I hover over the icon it shows the non-transparent UI color background.

  <-- This is the custom icon, I didn't import external images, just used the built-in ones

  <-- This is the native icon without customizing appearance: 

It's not a major issue but annoys me quite much whenever I hover over the icon. I watched tutorial videos of the previous versions like X4 and future versions like 2020 and they don't seem to have this issue. So I guess it's happening on X6 or later only? Why is this happening? Is there any workaround?

Any help is appreciated! :)