In the Channels tab, the Alpha Channel must activated.
Activated channels appear highlighted generally in blue in the Channels dialog (sometimes in grey). Inactive channels appear in white. If you click on a channel in the list, you toggle activation of the corresponding channel. Disabling the alpa channel has severe consequences. If you disable the alpha channels, all pixels from now on added to the image will not have alpha component, and so a pasted pixel will have the transparency of the layer on which it is pasted.
If the layer on which it is pasted is transparent, the pasted pixel will be transparent.
If the layer on which it is pasted is white, the pasted pixel will be visible.
This explains the behaviour you observed.