My question is if a Union Lawyer holds back evidence from his client which is damaging to his clients defense in a Arbitration case and then admits in the Arbitration Hearing he was well aware of the evidence is he violating his clent?
A lawyer will see mountains of evidence that their client doesn't see - one of the reasons you hire a lawyer is so that they can deal with the evidence so you don't have to.
Also, much of the evidence in an Arbitration case will be "damaging to his client's defense" - if everyone agreed on what happened and how the law works on what happened you wouldn't be in Arbitration.
Now, if the lawyer had admitted that they weren't aware of the evidence he might not be doing his job well.
If I understand correctly, your question can be reworded as:
Is it detrimental to a client if his attorney admits he concealed from Arbitration evidence that is adverse to the client's position?
Concealment and its subsequent admission can be detrimental to the client, and outlining a scenario therefor is not far-fetched. Whether or not it truly will be detrimental depends on the facts of the case, the evidence at issue, and the procedural rules which are applicable to arbitration proceedings in that jurisdiction (I will assume you have in mind a jurisdiction somewhere in the USA).
An arbitrator is authorized to subpoena witnesses or documents (Rule 30 of Employment Arbitration Rules and Mediation Procedures). It is foreseeable that some of the subpoenaed records may be entirely or partially protected by the attorney-client privilege.
If instead of properly listing evidence in the privilege log and/or reasonably redacting it the attorney conceals the evidence altogether, the privilege might be deemed waived. See EEOC v. DBO USA, L.L.P., 856 F.3d 356, 363-364 (2017). Interestingly, EEOC at 364 reflects that the attorney's omission can affect more records than just the one which was not properly listed:
see also Stafford Trading, Inc. v. Lovely, No. 05-C-4868, 2007 WL 611252, at *8 (N.D. Ill. Feb. 22, 2007) (treating an email that forwarded another email as two separate communications and holding that privilege was waived for both if either one was sent to an unidentified recipient).
In such cases, a waiver of the privilege might unearth additional, detrimental information that otherwise could have been concealed under pretext of the [retrograde] attorney-client privilege.
From this, a tortfeasor or criminal would conclude that it is best for his attorney to deny his concealment of adverse evidence. However, when that concealment is circumstantially evident or proved in some other way, it is in the attorney's best interest to refrain from staunchly denying his lack of Candor Toward the Tribunal (Rule 3.3 of Rules of Professional Conduct), lest he worsens his position in subsequent disciplinary proceedings.
There is a slight but realistic chance that a reviewing court might act with integrity and grant the substantive & procedural remedies in response to an attorney's concealment of evidence. In those instances, an attorney's admission of concealment could help his client in the form of damage control.