Per the whatwg standard, attributes has no default value in MutationObserverInit (i.e., not false).
Also, attributes should be set to true if implicitly true (i.e., attributeOldValue or attributeFilter is specified).

The observe(target, options) method, when invoked, must run these steps:

1. If either options’s attributeOldValue or attributeFilter is present and options’s attributes is omitted, then set options’s attributes to true.

This PR will allow the omission of the attributes option if attributeOldValue or attributeFilter is specified.

Depending on your setup, you can work around this in the meantime by patching the observe method:

const MutationObserver = require('mutation-observer');
const _observe = MutationObserver.prototype.observe;
MutationObserver.prototype.observe = function observe(target, options) {
  const needsAttributes = options.hasOwnProperty('attributeOldValue') || options.hasOwnProperty('attributeFilter');
  if (needsAttributes && !options.hasOwnProperty('attributes')) {
    options.attributes = true;
  }
  Function.prototype.call(_observe, this, target, options);
};
Object.defineProperty(window, 'MutationObserver', { value: MutationObserver });

(h/t @chandlerprall)