Ok, so I am not sure of all terminology and everything (pretty new to mustache still), but I was able to boil it down to a pretty small test case that shows the functionality

Express

router.get('/a', function(req, res) {
    res.render("A");
});
router.get('/b', function(req, res) {
    res.render("B");
});

A.mustache

{{<C}}
     {{$C}}
          A
     {{/C}}
{{/C}}

B.mustache

{{<C}}
     {{$C}}
          B
     {{/C}}
{{/C}}

C.mustache

{{>D}}
C: {{$C}}{{/C}}

D.mustache

D: {{$C}}{{/C}}

Now, load up /a and then /b, the first will output D: AA C: AA as expected, the second D: AA C: BB, now reload node.js and load up /b first and then /a and you get the exact opposite (D: BB gets 'stuck'). I have tried figuring this one out myself, but I got lost in the source code entirely.