How fix?
Check your drive belts, set screws and eccentricity bolts. You have loose hardware
When I burn with the first pass (no matter how many repetitions), the laser burns evenly.
When I start the second pass (again, no matter how many repetitions), I get the result shown in the photo. Are you sure about your words?
Given one picture and four words, I think @JimNM has nailed it.
This test pattern helps identify mechanical problems:
Scale it to fit the platform and run it as fast as it will go in Line mode with optimizations turned off and power set to mark a sheet of cardboard. Any differences from the design will be informative.
Upload good photos of the result so we can look over your shoulder.