The short explanation
The device reads the EDID from the display connected at its output and presents that same data to the source at its input. Nothing is added or removed.
Where you run into it
It is the right starting point. Before deciding anything is wrong with the capability data, you want to know what the display actually advertises — and copy mode both shows you that and puts it in front of the source.
If the fault disappears in copy mode, something further up the chain was rewriting the EDID. If it persists, the display's own EDID is part of the story, and modifying or replacing it is the next step.
How the DR HDMI 8K handles it
COPY TX SINK is the first of the DR HDMI 8K's three EDID modes, alongside CUSTOM EDID and COPY TX SINK EDID and MODIFY. With DisplayID support from firmware 1.3 onward, copy mode handles 384 and 512-byte EDID from modern displays rather than truncating them — which is exactly where older devices in the chain go wrong.
Want to test your own HDMI chain?
The DR HDMI 8K is sold through the HDFury.eu web shop.