Strange behavior in time hierarchy

I'm getting an strange behaviour.

We have a time dimension in SAP HANA, generated with the wizard.

In that table we have a range time since 1950 to 2050. Till there, everything has worked well.

When we create an attribute view with type "Time", and we use the default, or our own time hierarchy, and we try to visualize an Analytic View that uses the time dimenssion in Excel, there is a malfunction in the information representation.

I mean, with that simple hierarchy we have something like

Trying to fix that, I added the key DATE_SAP at the lowest level in the hierachy and it worked.

Why?

The documents that I've read didn't mention that.

Or, do you have any document that points you to add that key?

We are working on SAP HANA 85

Thanks in advance.