Describe the issue

1. here's a sandbox with a stacked bar chart with 2 annotations: https://codesandbox.io/p/sandbox/elegant-allen-tpqfxy?file=%2Fsrc%2FApp.tsx%3A32%2C33
2. The start time for both annotations are the same and they are nicely aligned.
3. However the moment you remove stackAccessors prop, they don't start in the same place anymore.

This task is important for Observability team(elastic/kibana#175513).