Most of these spindles are supplied and used unshielded and last forever, I wouldn't worry about that justCNCit unless you're getting interference issues (usually the controller fritzing out and movement stopping rather than the spindle). 12000RPM is also well above the usual danger zone for low RPM and cooling which is usually closer to about 6000RPM. Unless you're loading to the point where it's regularly stalling.

I'd put this one down to the usual dodgy QA letting through something with a bad solder joint internally and go with the replacement. Just in case, though, when the new one arrives and you start this job up again I'd be keeping a close, close eye on the temperature of the spindle through the first couple of runs. Maybe even lash out a couple of bucks for a cheap thermocouple and temp display to tape to the inside of the spindle cap so you can see if owt's going awry without putting your hand in the way of a moving machine to feel it.