Refresh token problem

I think this might be a tricky one to diagnose, but I suddenly seem to have a problem with my local site that I wasn't having on Friday.

When I log in to the site running on localhost, it makes a POST request to /api/users/refresh-token, which produces a 404 and then logs me out.

I'm using the next-payload module to run Payload within my NextJS site, and one of the last things I did was to move the public-facing pages of my site to the old-style pages directory (rather than app) and to implement a preview mode, via /api/preview, so either of those changes could have been responsible I imagine, but a couple of things are confusing me a little:

1. I can see a generated file under /pages/api/[collection]/refresh.ts but the request is for /api/users/refresh-token, so how does this match up?

2. The site is deployed to Vercel and it seems to work fine. That site doesn't make any request to /api/users/refresh-token at all

As I say, I think this might be a difficult one to solve as there seem to be a lot of variables, and probably I've misunderstood how Payload or NextJS works, but if anyone can suggest something to look at or explain why I might be seeing these things it would be much appreciated.