Did you sand the red area at all ? In case you just sanded the area between red and green this is the reaction where you did destroy the original paints most resistant point: the top surface, in the end, thats why you sand old paints top surface at all... so new the paints chemicals can penetrate and adhere.
However since you said: at the uncoloured area (nor red/green) the old paint already came off in flakes the reason is obvious:
The old paint already had hairline cracks, causing air and humidity to crawl under the paint, destroying the bonding of the old paint on the metal and it lifted in flakes. Now new paints solvents easily crouched under the old paints remains and lifted what seemed to be a tight and good underground.
Always strip/sand the areas where you don't have an intact paint to the metal generously. If its flaky its all shot and won't provide a good base. No need to completely strip all the case, but don't be too cheap
edit: No need to strip the case now completely to solve what you have either.
In case you sanded the red area, just sand the bubbled part down to the metal generously, smoothen with the other good paint parts ( 400grain) apply 2-3 thin layers to the new bare metal, blending over the already painted parts. Let it dry. Take a high grain wetpaper, like 600 to smothen the whole case another time then just apply an even thin or two layers over the whole case.