Eraser uses alpha/opacity of current colour
This might well be working as intended, but I find it a little odd.
Steps to reproduce:
* Select a colour
* Draw some stuff
* Reduce the alpha channel of the colour (not the opacity from the menu bar)
* Draw a some more stuff
* Right click an empty area of the canvas, or select the eraser
* Erase some of the fully opaque area you first drew, note how the eraser has the same opacity/alpha as the currently selected colour.
I would expect the eraser to be fully opaque unless there were some setting for eraser opacity (which there doesn't seem to be).