Correct answers were visible in a test; should I notify my instructor?
My guess is that it would be appropriate and pretty safe to note to your instructor that "some students" noticed that the correct answers were printed in bold. Sort of a "heard it on the grapevine" comment during an office visit. You could add "didn't notice it myself, hence the lower grade." 

The instructor will take some action or not, but you will have done your duty. 

You might, of course, know more about personalities that would make this unsafe for yourself, in which case, possibly let it go. 

But if the instructor doesn't know about it then it will probably happen again. 

I think this is a more serious issue in a medical related field, where people need to know things, not just get good grades. 

It might also be a judgement call as to whether this is "cheating". The others just noticed an anomaly.