>There's a problem to be solved, so let's attempt to solve it or at least compromise in good faith
Of course saying "I validate that you are feeling upset" is going to come across as patronizing and insincere. But I don't think that's because they validated your feelings. It's because of the way the validation is said.
Part of what makes a conversation good faith is hearing out what the other person is saying and agreeing where there is common ground to build from. That necessarily includes confirming the pain points each person is feeling.
Basically the difference between sympathy and empathy. You can validate someone's feelings by simply acknowledging them (sympathy, "I'm sorry you feel upset about that, how can I help?"), or you can participate in that emotion (empathy, "Yeah, that pisses me off too! Let's fix it.").
Neither is definitively better or worse, sincerity is paramount, and it's all contextual, including the personality of the person involved. I think aligning on what mix to use is possibly the most important thing in a relationship, especially a professional one.