@l446240525 -- Could you please provide us what the issue is, so that it would help us triaging the issue further.
Also please let us know the Chrome Version, OS Details and also the expected and actual behavior.
Thanks in Advance.

The problem is that a is not a containing block for b.  If you modify your css like this:

  #a {
    background: red;
    transform: translate-z(0px);
  }

... then the test case works as expected.  Details about what defines a containing block:

https://www.w3.org/TR/CSS2/visudet.html#containing-block-details