The biggest problem is not really that chatgpt was used but more than the conclusion is decided first and chatgpt is used to find the arguments to support it.

Normal, an expert should look at the facts and elements provided without a predecided result and forge his conviction based on the elements.

Obviously experts might be biased, especially when paid by the company, but it should still be in the understanding and evaluation of documents. Otherwise they are not an expert. They are not "lawyers" with the task to find incriminating or exonerating arguments.

Their testimony should start with something like "in my honest opinion". That is obviously not compatible with taking chatgpt to generate your testimony by asking to prepare arguments that support your customer.