teams camera aspect ratio weird behaviour

hello I am working on an iMac

installed sonoma 14.1.1 and latest version of new MS Teams (but same problem with classic)

I have a canon eos m50 mark II attached through an HDMI to USB converter (atomos connect 4K)

now comes the really weird behaviour

when I select this camera (USB video) in MS Teams the aspect ratio gets all squeezed up (also the camera in zoom, obs and photo booth) and then when I select this same camera in photo booth for example the picture gets unsqueezed again

so I guess there is something wrong with a driver or something but I don't seem to be able to find it (already reinstalled MS Teams but to no avail)

thx for any help

Pascal