Permalink automatically changing itself when using anything but 'default'

Unless I am using ‘default’ permalinks, whenever I edit any page on my site it automatically changes my link from whatever I have it set to to something else.

For example, if I’m using ‘post name’ permalinks and the page link is set to /what-we-offer and if I go in and update the page it will automatically change it to /about-us (not that specifically but it’s an example). The same thing will happen if I edit /contact-us. That is, on saving /contact-us it changes the permalink to /about-us. No matter what page it is, it will do it automatically.

I have tried deactivating all plugins, updating permalinks and totally refreshing .htaccess etc. but nothing has fixed it. Am I missing something? It changes it to the same thing each and every time.

Would so appreciate any help because this is driving me mental! Thank you!