You are in a subtract mode with your selection, check out in the Tool Options that you are in "Replace" or addition mode like in the screenshot below
If you don't see your tool options tab/dialog, go to Windows ➤ Dockable Dialogs ➤ Tool Options (is the first one from the list)
If this happens with the Bucket Fill, in tool options look somewhere there (don't use my settings) but look for fill whole selection, fill transparent...
https://docs.gimp.org/2.10/en/gimp-tool-...-fill.html
If you don't see your tool options tab/dialog, go to Windows ➤ Dockable Dialogs ➤ Tool Options (is the first one from the list)
If this happens with the Bucket Fill, in tool options look somewhere there (don't use my settings) but look for fill whole selection, fill transparent...
https://docs.gimp.org/2.10/en/gimp-tool-...-fill.html