In the UK English version of the Help on ‘Vector content defaults’ (or ‘Object defaults’ in Designer and Publisher) the word ‘synchronise’ and other words with the same root are variously spelled with ‘-is-’ or ‘-iz-’.
Photo: https://affinity.help/photo/English.lproj/pages/DesignAids/objectDefaults.html
Designer: https://affinity.help/designer/English.lproj/pages/ObjectControl/objectDefaults.html
Publisher: https://affinity.help/publisher/English.lproj/pages/ObjectControl/objectDefaults.html