keyboard swelled/keys cracked and stick when typing.

Hi there. My Macbook Pro keyboard was replaced under the Apple keyboard warranty program -

see here: https://www.forbes.com/sites/ewanspence/...

NOW the problem has occurred again 4 months after the keyboard was replaced by Apple. The keyboard has swelled and expanded causing the keys to stick when typing. It is frustrating and difficult to use a perfectly new and good machine. I have gone to Apple and they have said there is nothing wrong with the battery to cause swelling. They have given me a quote of close to 600USD to replace it.

My query is:

1. What would cause swelling of a keyboard?

2. Should Apple not be liable for replacing the faulty keyboard again since the problem has occurred again?

Please help!

Thanks.