Visualization issues when disabling mutisampling

I have created an OpenGL multisampled rendering context (I set 4 as the number of sub-samples per fragment).
It works fine when multisampling is enabled, but it has some visualization issues when I disabled it,
as noticed in this picture:

As you can see, it seems to work only partially: there are pixels which still seem combination of adjacent pixels, as if multisampling is still enabled in such portion of the screen (it can be noticed in the red circle).
The graphic objects rendered in my picture are just some GL_TRIANGLES, and a GL_LINE_LOOP.
Everything is rendered through the FFP, with lighting disabled.
I also tried to explicitly disable antialiasing locally for lines and polygons:
with no success.
That issue causes problems to my application, as I’m disabling multisampling for color-based picking of objects (each object is rendered in a specific color, used as an ID).

My PC is equipped with this graphic card (information gotten with glGet(…)):
Vendor : NVIDIA Corporation
Renderer : Quadro RTX 3000/PCIe/SSE2
Version : 4.6.0 NVIDIA 462.59
Shader version : 4.60 NVIDIA

I cannot see where my code could be wrong.
Any help is really appreciated.

Gianluca Natale