The problem is that paint color changes over time because of things like UV and oxidation.
So, the paint under a decal will be closer to the "new" color than the rest of the frame. If you've ever removed a picture frame, or furniture that's been there a long time, you'll have seen this effect.
Gentle wet sanding might help, but may be too aggressive. You may have better luck with automotive rubbing or buffing compound, which does a better job blending old and "new" paint colors.
BTW. Most never notice because the transition is gentle, but if you look closely at a bike's top tube you'll find they're different shades.
It won't matter until you carefully match touch up paint color.
Last edited by FBinNY; 04-02-24 at 04:05 PM.