Cloudwatch Dashboard overwrites metric aggregation period - how to stop that from happening

When I add a custom widget with a custom metric to a cloudwatch dashboard, I set the "period" to 5 minutes. After saving the widget, the period gets changed to 1 hour in the background and the displayed data is incorrect. My metric is published every 5 minutes and therefore if the aggregation period is more than that, then the result is incorrect.

This is how the metric looks like in the dashboard:

This is how it looks like if I click "Expand" and change the aggregate period to 5 minutes:

This is the same view, but with aggregate period of 1 hour, which is incorrect! The same values show up as by default on the dashboard:

And this is how it looks like in the "Edit" view. I have set the aggregate period to 300 seconds:

You can see why this is important, a difference in 99% and 95% uptime is huge!

Why doesn't Cloudwatch Dashboard respect the Widget definition and how can I force it to do so?

I tried:

1. Setting "Period override". But that only changes the period over which the metrics are displayed.
2. Changing the "period" value in the widget source, but that doesn't have any effect. It gets overwritten by the dashboard.

Does anyone have any additional tips on how I could get the behaviour that I want from the Dashboard?

Thank you and have a nice day!