This repo reproduces the error in the current SvelteKit version.

App in the repo has such routing

(app)
-- [id=bigint]
---- +page.svelte
-- +error.svelte

When you open the page with the parameter which is not bigint, you should see the +error page. But, instead, you see the default SvelteKit error page. Excepted behavior: +error page should be shown.