The material is acrylic and have engraved literally a few hundred units prior to this and not had the issue before.
Unfortunately yes, 2 variables have changed, the control board (mini GRBL) & lightburn.
These are the other topics that have come up with the same issue, but unfortunately no resolution:
This one has the most info - Dots appear all around engraving - #17 by Julien
Phantom dots engraving with Lightburn
Engraving disturbance littles dots - #7 by Julien
Little dots after engraving - #3 by EEVasquez
Random Dots When Engraving
So the ‘bubbles’ or dots are random. I’ve ran multiple jobs straight after, and it does randomise. The photos in the example, the dots came out first, then job ran a second time and didn’t come up. No changes to settings at all.
The reason I’m ruling out interference is that I hadn’t had this issue before with all the wiring, stock k40 wiring is all over the place anyway, but re-grounded everything when I first got the unit. Other people have ruled out interference as well with shielded wiring etc.
The best explanation for this is that for some reason the laser is being told to ‘fire’ for the tiniest amount.
There is a test I want to run, I’ve noticed that with fine engraving it’s a bit more obvious whereas thicker stuff it’s not. So going to try a simple test file to see what happens.