There are several factors causing / possibly causing this. The detail for these can become so granular, I have hesitated to chime in. I will be brief:
- Paper backed acrylic (usually is adhered with an adhesive). Peel that off before you engrave.
- Blasting with air assist. The best engraving of acrylic is using minimal air assist. Even better → none (but then you have to mitigate several other issues by having very good fume extraction, increased safe Z distance (space between the cone and the material), and control for when the job goes from engrave to cut
- Your speed. Too fast a mess is made while acrylic snow showers the landscape. That last “lap” clear result may actually be in every line. The “snow storm” from the next line may be covering it up line after line (while the last line has no tailing storm of acrylic dust). Too slow and instead of frosting you’re polishing
- Lens focal length
- Acrylic type (not just cast vs extruded. there are various formulations of each)
- Line resolution