I’ve just tried to get my BM Decklink to work with 5.5 and I’m still having the same issue since 5.3, forcing me to stick with 5.2.
When using a Decklink as a BM media input, it works fine. Unreal sees the video from the SDI. I can display the video in the Media Player, created when I made a Media Bundle. However, the Media Texture creates a black, 2x2 pixel texture. Dragging a Media Bundle into the scene just creates a black plane with no video feed. Creating a Composure layer made up of several CG layers and a media layer, the media layer is black.
What has happened Epic? This worked without fault in 5.2, but it seems you have broken it for any subsequent releases - 5.3, 5.4 and, now 5.5. 
The Decklink 8K Pro is on your list of working hardware, but it isn’t working as it did and, from reading other threads, it’s not just the Decklink 8K Pro. Other BM cards are having the same issue. Please, sort it out.
Thanks.
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