I was told that if this happens to brushless motor, it means that magnets inside has been demagnetized. This could be due to faulty material or aging of magnets. In your case for sure isn't age. I was also told that this has an affect to efficiency of the motor - it will heat more than it would if magnets were OK. This could lead to permanent motor damage.
From my personal experiences: I have 1 brushless motor on one of my RC buggys, on which you couldn't feel any magnet resistance when turning it by hand after a few months. Believe it or not, but this motor runs - from performance point of view - better than my other motors on the exactly the same type of buggy and with exactly the same type of ESC, same settings, gearing,...
But it does heat for app. 20% higher and
battery lasts for app. 20% less.
Spark will fly with this motor. But if I were you, I'd do a test flight and compare motor temperatures of all 4 motors. If this motor is way more hot than other 3, I'd replace it.
I have no worries for my RC buggy: if motor will burn out, buggy will stop and that's about it. If motor fails on Spark, it will fall from the sky like a stone.