OALD has the entry for both! Furthermore, it's the same context
For the adjective bitter
(of arguments, disagreements, etc.) very serious and unpleasant, with a lot of anger and hatred involved
And, for the adjective heated
(of a person or discussion) full of anger and excitement
The sentence in concern uses both the phrases which triggers my mind and I can say that they just wanted to include everything i.e. anger, unpleasing and seriousness. Because had they used merely heated discussion, a chunk of readers might dismiss unpleasing as the meaning.
However, I see both quite similar. If something includes anger, it's quite unpleasing. Isn't it?
[Good to note that the adjective bitter can be used for anything that is unpleasant. Bitter truth is the commonest phrase to mean the truth which is not-so-likeable].