Consider two graphs loaded from the following Turtle strings:

[ :p 
    [ :p :X ] ,
    [ :p :X ] ,
    [ :p :Y ] ,
    [ :p :Y ]
].

[ :p 
    [ :p :X ] ,
    [ :p :Y ] ,
    [ :p :X ] ,
    [ :p :Y ]
].

They differ only in the order of URI object nodes of the blank objects of the first blank node (XXYY vs XYXY).
They both consist of the following statements:

Or visually:

Yet they're not equal:

var g1 = new Graph();
g1.LoadFromString(@"
@prefix : <urn:> .

[ :p 
    [ :p :X ] ,
    [ :p :X ] ,
    [ :p :Y ] ,
    [ :p :Y ]
].
");

var g2 = new Graph();
g2.LoadFromString(@"
@prefix : <urn:> .

[ :p 
    [ :p :X ] ,
    [ :p :Y ] ,
    [ :p :X ] ,
    [ :p :Y ]
].
");

Assert.Equal(g1, g2); // fails

I've traced things as far as learning that GraphMatcher resorts to brute force mapping for this topology, which probably explains the abhoring growth rate:

But it doesn't explain why they're not considered isomorphic.