amp-mega-menu occasionally fails to render

## What's the issue?

The amp-mega-menu parent header tag's class sometimes erroneously does not get the "i-amphtml-mega-menu-mask-parent" class. This may be a race condition.

I have been able to increase the rate of failure by using "display: flex", but it does not appear to be specifically related to that CSS property.

No workaround appears to exist.

## How do we reproduce the issue?

A min-reproducible test case is available on jsbin: https://jsbin.com/ripahibuza/edit?html,output

Refresh multiple times to see two different rendered outcomes:

Observed, expected:

Observed, erroneous:

## What browsers are affected?

This min-reproducible has been tested against the latest Chrome 86 on Windows 10. The issue has been observed in other environments as well.

## Which AMP version is affected?

This does not appear to be a regression. It may make sense to review #27454 when resolving this issue.