Strange audio behaviour with NDI

Did this week run in to something very strange that I don't understand why or how to fix.
I have a system with vMix (z420, 2x Decklink Quad, vMix v20) and a standard CasparCG server(HP z420, 2xDecklink Extreme, Caspar 207) in a network. I wanted to run a video clip from Caspar on vMix over NDI. According to mediainfo the file was 1080i5000 MP4, h264, 48kHz stereo. When I play the file on Caspar the system audio is OK, the embedded SDI-signal is OK, but in vMix it says that the audio is 44kHz, 32bit and plays at half speed.
So I suppose the audio in the MP4 file is 48kHz 16bit, but somehow the bit information goes wrong in NDI.
Has anyone else had the same experience?
I'm really confused because I dont seem to find any videoconverter that would give me an option to change the bit.
And I know the correct workaround is to play the file i vMIx (where it plays out correctly) but I would love to know what and why this happens, is this a CasparCG, vMix, NDI or a iVGA problem?

br markus