From an employee pov, it is right to know when work given is too much. Teaching something you not familiar with with can lead to more problems for both the teacher and the student. I’m not sure how the situation is at your work, its ok to feel stress and burn out. Some places have avenue for secrecy and stress reporting. It is also direct violation of contract for putting you in area you are not suppose to. If you don’t feel growth, its good that you find somewhere that you can grow and persue what you want
From management point of view, giving challenges helps growth. However, seeking the one approval beforehand instead of just assign, is an important factor. The management should have asked if you are willing to take the challenge. Even if you don’t take it up, it should not divert you from the job you were suppose to do, via contact. A good manager should have asked if you wanted to grow in scope of work, or depth in your field, or both. Even if from their point of view, you are running away, it shouldnt be vocalize out, it makes the employee lack trust in the manager, not just the indiviual but also the others on staff level. Their disappointment shouldnt make other party feel guilty. The guilt that you feel, if what you’ve done is wrong, shouldn’t have happen in the first place.
Maybe there’s manpower issue which require more workoverload from other stuff, but that is the manager job to look for more people and not over work the other stuff. The manager is suppose to stand in and cover the work as he stands above them, and if he is unable to, he should find someone who is willing and suitable. Constant is important. Even if a group of outstanding staff, if the manager can’t help grow and develope them furthur in the direction the staff wants, its a failure of a manager.