The eraser only erases one layer of contast

[calixtus06]

Hello I am new on this forum.
I'm used to using gimp ( 2.10) with linux and here appears a problem that I can't solve on one of my computers: when I use the eraser it erases the contrast of the image and the image gets lighter. Usually I have either the image behind or the background appearing depending on whether I have added an alpha layer or not.

thanks for your help!

Nico ( france)