Fable is on a relative basis not great at writing prose and talking like a human.
One possible reason: coding tasks are long-horizon, so most text output is a means to an end.
Most text generated by Fable is never seen by a human.
So it's drifted from a dialect that is more human shaped to one that is more whatever emergent shape helps produce good coding results.
As others have called it, "Claudeish".
Claudeish starts off as a dialect of English, but at this rate it will diverge pretty soon…
In the engineering context, "seam" is a very Claudeish word that has now become more common in English, too.