Netlify redirect rule breaking NextJs

Question

I'm building a NextJs site hosted on Netlify. The idea of the site is that it will be a gradual replacement of another site built on older tech, so the NextJs site will return a page if it has it otherwise the request will be re-written to the older site.

I have a Netlify redirect rule setup in the netlify.toml file as follows:

[[redirects]]
    from = "/*"
    to = "http://old-site-url/:splat"
    status = 200
    force = false

For the most part this works. If I add a static file to Netlify, then it is returned, the homepage in NextJs is returned, and pages I haven't yet created get rewriten to the old sites url. However every call to anything in the /_next/ folder return a 500 error.

Remove the redirect and it all works again.