"The luck of a main character was often, unbelievably good."
I didn't know luck could be often. I wonder what often luck is?
I guess it's a synonym for 'good' in context. Quick reversal: "The luck of a main character was good. Unbelievably often." Hardly superb diction, but it works.
This is exactly the kind of mistake that computer-based grammar is
likely to make. It will have trouble identifying sub-phrases, because (like the author using it and the readers reading it) it doesn't actually know what words mean. It has to identify a phrase based on how it's spelled. The author doesn't know what commas are for, they have to figure out which commas go where by imitation, which is inflexible &c.
As I said earlier, this may be an accent. Maybe in their native language putting a comma there doesn't make you sound like a stuttering idiot child. However, at best, it's a grammar mistake that Grammarly doesn't fix.
That, said, there's this, one guy who, puts so, many unnecessary, commas that while, I'm exaggerating a lot it, rapidly, becomes, unreadable. This is the kind of problem that's caused when an insecure writer trusts a tool that's dumber than they are.