A very quick bit of searching suggests this is a well known problem. Here's two of many possible links.
https://www.justanswer.co.uk/ford/fdy1c-water-leaking-ford-focus-spare-wheel-well.html
https://www.fordownersclub.com/forums/topic/141526-water-in-boot-on-2017-ford-ka/
Cgdgji
September 2025