What is here the problem machine 100% letters do not engrave some places less
when working with plastics workpiece and machine rigidity will have a factor on finish like you’re seeing as will bit type.
What kind of bit are you using for this?
Also will want to ensure the material is held down well, double sided tape below will help wit this to prevent any bouncing up/down.
I think with Z Axis has problem
Starting depth not correct