<p>I'm printing with a Prusa MK3, with the following settings:</p>

<ul>
<li>3 perimeters</li>
<li>50&nbsp;% infill</li>
<li>infill overlap: 50&nbsp;%</li>
</ul>

<p>The filament is Polyalchemy emerald green (PLA). Nozzle temperature: 210&nbsp;°C.</p>

<p>On a simple part (it's a keychain), the shell detaches if I apply a bit of force on a zone of the part that is "fragile". See picture. You might not be able to see it, but only the 2 external perimeters detach from the rest of the part. I used to print this part on another printer, and I never observed this problem.</p>

<p>Any idea on how to solve this problem? It seems the 3 external perimeters didn't fuse properly.</p>

<p><a href="https://i.stack.imgur.com/cUI0O.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/cUI0O.jpg" alt="enter image description here"></a></p>

I am looking at your picture, and I realize that it is not only that the perimeter is detaching from the infill, but also that the perimeter is breaking.
Once the perimeter breaks, the weaker connection with the infill will surely break, too.
You could try:
Using more than two perimeter layers.  Go big. Try five.
Add a fillet where the ring attaches so that the force is not focused on a point.
Choose an infill percentage and pattern that maximizes the contact between the perimeter and the body.
Change the angle on the bed at which the keychain is printed so that the infill maximally connects with the breaking point.  That might make the ring be 45 degrees off the X and Y axes.