If Personalchefs were the subject, the verb would have to be in plural, because Personalchefs is plural.
Also, if the bold part were accusative, it would have to be einen Berufs- oder Personalwechsel, because the accusative masculine singular form of ein is einen.
In this case the sentence would be:
Personalchefs stören einen Berufs- oder Studienwechsel der Bewerber nicht so sehr.
But this would mean that the head of HR won't interfere with a change of profession or field of study, which is obviously not the intended meaning.
who is not bothered by what/whom? who= personalchef= subject
Well, here you used the passive voice (being bothered) but the German sentence above uses active voice. So the Berufs- oder Studienwechsel (subject) bothers the Personalchef (object).
You were probably lead astray by the fact that the head of HR is the first part of the sentence. But remember that the subject does not have to be first in a German sentence. It's also possible that the object is the first part.