Wood and MDF are two different things. Can’t say for sure, but it looks like you are engraving on a wood veneer. You will never get even finishes with light engraving on wood. It’s a density problem.
that looks like a physical problem with your diode timing.
Possibly it’s an electrical fault, but more likely an electronic one with an overheating diode or driver.
It’s not material-related at all. If you ran the same job on paper or wood or MDF, at the same speed settings, you will get the same result. The diode is cutting out.
You can see that the start of the burn is the same, it’s cutting out past the start point.