Redirects applied when under a prefix set by a reverse proxy lose their prefix

[cap-mfm]

Summary

Hi, I have an issue where I have sites made with Next.js that are served by nginx as a reverse proxy, each one under a different prefix.

The issue I'm encountering is that the redirects that Next.js is applying when removing trailing slashes (I have set trailingSlash: true in my config) get rid of the site prefix ending up in 404s.

In my config i have set trailingSlash: true and assetPrefix: '/abcd', as I can't use basePath because it'd make Next.js files get served under a double prefix for some reason (e.g.: /abcd/abcd). It all works well so far, but when users hit an URL that lacks the trailing slash, the redirect that gets applied removes the site-wide prefix like so:
https://my.site/abcd/example-page -> https://my.site/example-page/

This has been reported in several issues before (e.g. this one), and I can't seem to find any solution working in Next.js-land so far. Is there any stable solution that I'm missing, or is it something that could get get fixed by Next.js in the future?

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