What is the issue with the HTML Standard?

I encountered this investigating why Gecko crashes in navigate-multiple-navigation-navigate.html. Gecko implements this assert as assert in the code, which gets hit when navigate #3 gets called.

As I understand, the assert is hit because the navigate to #2 aborts all ongoing navigations, which aborts navigate #1, which then synchronously runs navigate to #3, which maybe creates a new upcoming API tracker, which asserts because there already exists one (the one for the navigate to #2). This all happens before promoting the upcoming API tracker to ongoing in the inner navigate event firing algorithm.

cc @noamr @zcorpan @farre