The dot-matrix is likely being painted under your shader canvas or washed out by mix-blend-mode. Here’s a bullet-proof layout that guarantees the matrix shows up above the shader and below the UI.
The background should feel like the faceplate of a precision instrument — a dark, brushed titanium or anodized aluminum panel where each dot is an LED cell beneath glass. The surface subtly reacts to sound, light, and interaction, like the living skin of a synthesizer.