All posts appearing in parent category they're not assigned to

My blog is all about city breaks, so I have a parent category set as ‘citybreaks’. I’ve just noticed that all of my blog posts are appearing in the ‘citybreaks’ category page on my website (https://thiscitylife.co.uk/category/citybreaks/) even though in the back end they’ve not been assigned to that category. Only 4 posts have been assigned to ‘citybreaks’.

How do I stop all of the posts showing up on the website? As the main subject of my blog is city breaks and every now and then I post a random subject, I really need to make sure posts about other subjects aren’t being lumped in with city breaks!

Thanks

The blog I need help with is thiscitylife.co.uk.