The checker pattern indicates transparency.
Your work-flow is a bit strange but anything you paste into the decomposed alpha = various degrees of opacity The alpha channel is a mask where Black = transparent, White = opaque, Anything in between = degrees of semi-transparent.
Some image viewers will show transparency (usually as a checker pattern) but some do not.
You can get the same effect if you add a layer mask (white) Layer > Mask > Add layer mask to the first image and paste the second image into that.
Want to lose the pattern ? Get rid of the alpha channel Layer > Transparency > Remove Alpha Channel .
Need the alpha channel but not the pattern, It can be set to white in Edit > Preferences > Interface > Display then the Transparency Check style drop down menu
Your work-flow is a bit strange but anything you paste into the decomposed alpha = various degrees of opacity The alpha channel is a mask where Black = transparent, White = opaque, Anything in between = degrees of semi-transparent.
Some image viewers will show transparency (usually as a checker pattern) but some do not.
You can get the same effect if you add a layer mask (white) Layer > Mask > Add layer mask to the first image and paste the second image into that.
Want to lose the pattern ? Get rid of the alpha channel Layer > Transparency > Remove Alpha Channel .
Need the alpha channel but not the pattern, It can be set to white in Edit > Preferences > Interface > Display then the Transparency Check style drop down menu