Next.JS 15 RC freezes occasionally when forwarding (`rewrites()`) requests to FastAPI endpoint

### Link to the code that reproduces this issue https://github.com/reasv/panoptikon-ui/blob/master/next.config.mjs ### To Reproduce I do not have a minimal or reliable way of reproducing this issue as it seems to happen inconsistently, but this behaviour seems clearly incorrect to me, and perhaps we can isolate the problem causing it. Above, I linked the configuration that seems to reproduce the issue with my FastAPI server, specifically the endpoint serving thumbnails: https://github.com/reasv/panoptikon/blob/master/src/panoptikon/api/routers/items.py#L200 ^This is the line of code returning the response that occasionally makes Next.js hang up. As you can see it's just a file response. When forwarding to this endpoint using the config I linked in the main link, the issue occurs, occasionally, and not consistently with the same files/same requests. The FastAPI server itself prints this error: ``` RuntimeError: Response content shorter than Content-Length ``` Which led me to blame FastAPI at first, but then I realized that fastAPI itself remained responsive throughout, while Next.JS seemingly freezes, often not serving *any* requests. Maybe it is an error caused by FastAPI, but this doesn't seem like a robust way of handling it. ### Current vs. Expected behavior I expect Next.JS to forward my requests to the server's API and return the responses. I am using forwards to avoid cross-origin requests and to allow for *runtime* configurable API URLs, through serverside-only env variables, which otherwise would have to be baked into the build. This works most of the time using the forwards provided above. Unfortunately, at seemingly random times, the thumbnail endpoint, which returns image files, seems to have issues. The image either does not load on the client, or it half-loads with the remaining part being empty. Next.js eventually prints ECONNRESET errors, but that happens after a *while*, in the meantime, next.js becomes varying degrees of unresponsive, to the point o…