Straight polygons have very visually long straight edges and sharp corners. One can increase the fractalization level to reduce the length of the straight lines, but unless you fracatalize a lot, it is very visible.

Smooth polys doesn't have a single straight edge or single corner, it is all curves. This makes a huge difference to the map, because all those straight lines and corners make it look very artificial, while the softer shapes of the smooth poly makes it much more natural.

If you have applied edge fade, it evens out the difference a bit, especially since edge fade will turn those corners into more rounded shapes, and can sometimes be just as good as alternative, but it depends on the map and the settings.

Both these maps could probably benefit from using smooth polys rather than straight ones for the various elements, such as the lava and river and rocky patches....

Not every annual has those, but you'll find the files in the root of the CC3+ data directory, named SUB20xx_Content.fcw

CC3+ doesn't install anything that even integrates with explorer, so I am really struggling with coming up with any case where this would be caused by CC3+. Certain 3rd party programs that changes explorer behavior, like Sue mentioned, can certainly cause this behavior, but there are no components installed by CC3+ that explorer would refer to, so when CC3+ isn't running, it is like it doesn't even exist as far as explorer is concerned.

The only "integration" that exists is the file type mapping that causes explorer to recognize the .fcw file type, displaying the right icon, and allowing CC3+ to start when you double click the file. But that is core explorer functionality, and all CC3+ really does is tell it where the executable is by giving it a text string, it isn't an actual add-on to explorer itself, and CC3+ does it exactly the same way as any other program.

When it comes to Windows 11 and Lenovo Legion Pro, I have that combination myself (A Lenovo Legion Pro 5i [2023]), and CC3+ works perfectly fine on it.

When you did your clean reinstalls of Windows, did you install anything else before CC3+, like a custom virus checker or something? (I have to admit, that 6 month trial of whatever Lenovo shipped with my machine went out the window almost immediately, I prefer Microsofts own, and I certainly don't want something I have to start paying on in a few months). Most antivirus software works absolutely fine with CC3+, but I am just trying to find some commonalities that could be a probable cause.

If you have access to Acrobat Professional, it may also be worth trying to feed it through the PDF optimizer, flatten the structure of the file.

Those old PDF's are pretty cool, because they contain actual vector data from CC3+, making them have the same "infinite" zoom capability that the original CC2 map it was made from. But unfortunately, that also means it is highly complex, which is why it probably crashes your readers now.