A strange phenomenon in the Sandmeyer reaction

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Has anyone carried out the Sandmeyer reaction? I encountered something strange, so let’s discuss it: I performed the classic Sandmeyer chlorination reaction, using cuprous chloride as a catalyst. The reaction proceeded normally at low temperatures, but when nitrogen was released at higher temperatures, a phenomenon different from the usual one occurred. Normally, the material released upon heating is light green in color (which is the color of cuprous chloride) and is quite viscous; this property remained unchanged throughout the heating process. Moreover, since many bubbles are formed during nitrogen release at higher temperatures, this time the material turned black after heating and became less viscous. It seems like this wasn’t the normal behavior. However, when cold water was added, it returned to its light green color, and the yield was not affected. I’m really puzzled – why did the color change? The reaction mixture remained strongly acidic. Please help analyze this