The romance lasted 'a few years,' a source tells ET.
ET has also learned this producer also worked with Robach at the time. The producer left GMA in late 2017. ET has reached out to Holmes, the former producer, ABC and GMA for comment. The Daily Mail and Page Six were first to report the affair.
When it comes to Holmes and Robach's romance, which a source previously told ET that has been "going on in secret for quite some time," two sources previously told ET that the duo would not face disciplinary action because they are two consenting co-workers, who are equals, in a relationship.
It's unclear if the same can be said about Holmes' relationship with the former GMA producer.
"There has still been no word from management addressing their relationship, but that's all anyone can talk about around the office," a source said of Holmes and Robach. "When and if they will ever address the elephant in the room, it's still unclear."
As for Holmes and Robach's respective marriages, sources previously told ET that Robach and her husband, Andrew Shue, as well as Holmes and Fiebig, had split before the new relationship became public knowledge.
Holmes did not wear his wedding ring on GMA after his relationship news broke, and Robach hasn't sported the accessory on Instagram since October.
The next month, Shue and Robach sold the New York City pad they purchased in 2018, ET previously confirmed. When Robach and Holmes' relationship was made public, Shue removed posts featuring or having to do with his wife from his Instagram account.
"It's been rumored that Amy and Andrew have had issues over the years so no one would be surprised if a divorce is in the works," the source said.
"The reality is [Amy] and T.J. have always had chemistry, so this match makes sense," the source said. "Also, Amy and T.J. are both adults, making adult decisions, who happen to also be in the public eye, so they're under a magnifying glass that isn't particularly fair. They both are together, willingly and that's all that should matter."