10W laser engraving issues / different contrast

I am engraving (laser fill) text onto Bambu black acrylic. The first batch are all consistent with high contrast, white on black, where engraved.

With the same settings and in the same project I get a significantly reduced contrast.

It seems related to specific parts and text parts.

Is there any settings to be applied per part in a single project? Can you control different degrees of engraving within one project and object?

Your help is appreciated.