How can I avoid the outlines of removed colors?

[Zydelum]

Hello,

With Gimp, I select one or more colors by color > Color tab > Color to Alpha.

Starting with this image:
[attachment=13626]

I can eliminate the yellow and blue without the outlines getting in the way:
[attachment=13624]

From another image:
[attachment=13623]

I can remove the yellow and red, but the old outlines remain:
[attachment=13625]

With Gimp, I can no longer reproduce the first technique and avoid the second technique.

Can someone explain why?




Thank you for your help.