Speer described it in depth in his biography. Their relationship was not public, and also not very open in the hidden corridors of the Reich. They slept in separate rooms even when away from the public. A girlfriend or usually has or even requires some degree of open recognition. Why? It was said they wanted it to appear that Hitler was married to the Reich. Germany came first and other such PR concerns. So they kept it a secret, so that's one factor.

But in Germany, she can and is referred to as his Geliebte which can mean mistress or lover, or also his Lebensgefährtin, which means companion or partner, also Ehefrau, which means Lawful Wife, or just Wife. But they were only married for a little while, so wife is rare, mostly because the sources we read are referring to them in company in the time before they were married.

Mistress doesn't necessarily mean they're involved with a married man. It only means that she's involved with a man -- and they're not married to one another -- and it's usually something in secret, as was the case with these two.

I personally think "girlfriend" is too childish in regards to Hitler, and "partner" somewhat ambiguous a term, so I'm not particularly shocked by "mistress".

I believe "mistress" was the term used for unmarried couples at the time, but an actual historian might have to confirm this. In the late 19th century classic French books I've read, they use the term "mistress" for what we'd call a girlfriend today.