Chained exceptions truncated when log level is set to TRACE

[zhixing0]

We find that when log level is set to trace, chained & unhandled exceptions (that reach uvicorn) will be truncated to the last exception. Below is a minimal reproduction script:

from fastapi import FastAPI
import uvicorn

app = FastAPI()


@app.get("/")
def root():
    try:
        raise Exception("E1")
    except Exception as e:
        raise Exception("E2") from e

if __name__ == "__main__":
    uvicorn.run(app, host="127.0.0.1", port=8080, log_level="trace")

Accessing http://127.0.0.1:8080 will trigger the exception, but only E2 will appear in the log.
In contrast, setting log_level to anything higher than trace (e.g. info) produces a complete trace with both E1 and E2.

The cause seems to be this MessageLoggerMiddleware introduced here when level is trace or below (line link).
Inside the middleware, a raise exc from None statement drops any chained trace (line link).
Is there any rationale behind this, or is it a bug?

FastAPI version: 0.111.0
uvicorn version: 0.29.0