any chance of photographing those flames? The main issue that comes to my mind is focus- if a fat beam is hitting the Romark it's going to do more heating than vaporizing, which may cause a blowtorch effect...

could be a beam-split issue but not likely- I was going to say lens issue, but I have one lens with a burnt spot and one that's cracked in 3 places and both of them still engraving fine!

you should maybe try a ramp test with a 10" long piece of Rowmark, I'm thinking your focus is in need of adjusting--

And what focal length lens are you using? is it possible it's not what you think it is?