In my experience clients will always be doing this, no matter what. They do know and realize what the contract may say but they won''t hesitate to keep asking things for free.
During my career I've encountered myself with clients that once the final project is finished keep asking you for new features from time to time, expecting them for free.
I've even found clients that contracted the services of other agencies/freelancers that messed up the initial design, and came back to me in order to fix those errors... guess what, yes, expecting me to fix them for free.
So in my humble opinion, you just have to gather patience and keep on. The web design world is pretty much a stressful are to be in, so you really have to enjoy what you are doing and keep on surpassing those "toxic" clients.
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Light up your journey