Mask Images

I'm new in Fusion and having a lot of trouble with masking images in Fusion.
Somehow my issues are so strange, that I don't even know if I'm doing something wrong, or is it a bug in fusion.
However, I'm trying to mask a large image (PNG File). The Mask also is a PNG-File with transparency.
The channel of the Image that has to be masked is set to "Alpha".
I connect the nodes, and the mask work, but somehow the larger the masked image is, the less I can see of it (see image1 below).
In other words: it looks like part of the masked image is cropped away.
Now what I find interesting is, that inverting the mask works without any problems(see image2 below).

How can I fix this problem?

I would try merging your image you are using as the mask with a Background node with its alpha turned to zero so it's transparent. Make sure the Background node is the same dimensions as your MediaIn3 of your example.