What is HDMI matrix switch?

In plain language

An ordinary switch picks one of several sources for one display. A matrix routes independently: a 4x2 matrix takes four sources and feeds two displays, each showing whichever source it likes, simultaneously.

Why it matters in your setup

A matrix creates an EDID problem it cannot fully solve. Each source needs a single EDID, but it may end up on either display, and those displays rarely have identical capabilities. Which EDID should the matrix hand out?

Every matrix picks a strategy: copy one output, use a fixed internal EDID, or merge the outputs down to their common denominator. All three are compromises, and all three explain the same complaint — a source that offers less than either display can actually do.

Add the fact that switching triggers hot-plug events and re-negotiation, and a matrix becomes the single most common place for capability data to go astray.

What the DR HDMI 8K does with it

Placing a DR HDMI 8K between a source and the matrix gives that source a fixed, known EDID regardless of what the matrix decides to advertise, and regardless of which display is currently selected. The source stops renegotiating on every switch, and you choose the capability set instead of accepting the matrix's compromise.