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The behavior you describe is quite odd. Usually, when you create two localized versions of a page, which are synchronized, the option to stop their syncing is available on the top of each language version of the page. Can you please make sure that you chose to synchronize the problematic pages when you first created them. I'm saying this, because you are also allowed to simply copy the content of the first language to the second one and keep the pages unsynchronized and in such case you won't have the option to "stop syncing" the pages, simply because they won't be synchronized. Please take a look at the attached image for further reference.
If the issue persists, please provide us with backend credentials to your website, so that we can observe this behavior further. Thank you for your cooperation!
Another way is trough sitefintiy pages API, this is done by the property LocalizationStrategy:
PageManager manager = PageManager.GetManager();
PageNode page = manager.GetPageNodes().Where(pN => pN.Title ==
PageData pageData = page.Page;
pageData.LocalizationStrategy = LocalizationStrategy.Split;
Please try to split your pages by code and see if this will work.
I have the same problem. I tried removing the whole language, but when I added a language it remembered some of my settings. Example one of my parent pages reappeared as soon as I added the same language (UK). And when I tried adding a new page it did not give me the option not to sync or not sync. It just creates a copied page.