Is it standard protocol for a probation officer to disclose who specifically reported violations to the felon?

No. That is not standard. In fact I might argue that it is improper.
Whether or not it is "illegal" I cannot say but I can tell you that you are 100% free to file a formal grievance against the PO with her/his agency.
You are 100% correct in asserting that you may have been intentionally (at worst) or recklessly (at best) placed in harm's way and any reasonable person would fear retaliation.
I hope that I have been helpful in answering your question.

Under the constitution a defendant has the right to confront their accuser in court and if the defendant wants to they can force you to appear at a deposition to take your testimony or cross examine you at trial if the State uses you as a witness to prove the violation charged.

A person who is accused of a crime has a right to face their accuser, that's been the law of our land since before the founding of the American Republic and it is preserved in our constitution. The officer did not witness the violation, they are taking the statement of the accuser, which is you. They are required to disclose who the witnesses are.
A violation of probation is a crime unto itself, different from the crime they are on probation for. If the state has to prove the violation and the officer did not witness it, they will be forced to call you to testify at the VOP hearing as to the violations which you witnessed.
This is same as if you witnessed someone rob a bank and then you tell the police who and what you saw. You are the witness, not the officer. If there was to be a trial you could be called for a deposition and you would be called for trial to give your statement under oath to a judge and jury, with the accused looking right at you.

You fear retaliation. It is a crime for the defendant to try to get you to change your story, it is witness tampering and it is a third degree felony. In my opinion this can be done directly or indirectly.

There is nothing prohibiting it and as a matter of due process, the probationer has the right to know who is accusing him of the violations. Otherwise there would be no way for the probationer to challenge the accusations of violation of probation.