If vercel handler is served under a path like /blog/ inside nginx then trailingSlash option redirects will redirect to top level domain name

[GrosSacASacs]

serve-handler/src/index.js

Line 155 in c7a4043

 target = `${decodedPath}/`; 

The solution is to use relative redirect links eg
Location: ./folder/
instead of
Location: /folder/

[GrosSacASacs]

with the trailingSlash option