Hyperdeck Mini alpha channel choppy on Atem Mini

Hi everyone, I am new here.

I have an issue with alpha channel playback from hyperdeck to atem mini. It is choppy.

I have the following setup:
Hyperdeck Mini → 2 SDI cables → 2 BMD Micro Converter BiDirectional SDI/HDMI 3G → 2 HDMI Cables → Atem Mini Extreme ISO HDMI 7 & 8 (all cables have exactly the same length)

Now if I play any file with an alpha channel, I get choppy cutouts. However, it is random, some alpha is rendering fine, even with gradients, but some parts are just blocky. Setting the key to Pre Multiplied Key does not work at all, changing the gain and cutout levers there is no setting that makes it any better.

I tried almost all formats in Adobe Media Encoder. If I overlay the file (Prores 4444) over a Video in Premiere, it looks perfect.

I have no further Idea where I could be looking. Anyone has an idea? Is it the SDI cables? The converters?

Thanks for reading, Thomas.

this is how it looks like in premiere using the same file:

and here is another part of the choppy sample: