To me similar happened in 4.24 and 4.25 too.
But all the requests under /content-manager/single-types/api::* were giving 404 not 200.
I have added a workaround here:
Basically need to change single type route handler to collection type handler:
/node_modules/@strapi/plugin-content-manager/dist/server/index.js line around 160:
Change handler to “collection-types.find” after which they will become visible and you will need to fill at least one field with data so they will remain visible with “single-types.find” handler as well.