I'm also seeing this.
My output,
ubuntu@ubuntu:~/rclone-v1.55.1-linux-arm64$ rclone -vvv serve http --addr ':9097' ./
2021/07/28 03:35:18 DEBUG : rclone: Version "v1.56.0" starting with parameters ["rclone" "-vvv" "serve" "http" "--addr" ":9097" "./"]
2021/07/28 03:35:18 DEBUG : Creating backend with remote "./"
2021/07/28 03:35:18 DEBUG : Using config file from "/home/ubuntu/.config/rclone/rclone.conf"
2021/07/28 03:35:18 DEBUG : fs cache: renaming cache item "./" to be canonical "/home/ubuntu/rclone-v1.55.1-linux-arm64"
2021/07/28 03:35:18 INFO : Local file system at /home/ubuntu/rclone-v1.55.1-linux-arm64: poll-interval is not supported by this remote
2021/07/28 03:35:18 INFO :
Transferred: 0 / 0 Byte, -, 0 Byte/s, ETA -
Elapsed time: 0.0s
2021/07/28 03:35:18 DEBUG : 3 go routines active
and here's the output from v1.55.1 that I downloaded to test I wasn't going crazy,
ubuntu@ubuntu:~/rclone-v1.55.1-linux-arm64$ ./rclone -vvv serve http --addr ':9097' ./
2021/07/28 03:34:43 DEBUG : Using config file from "/home/ubuntu/.config/rclone/rclone.conf"
2021/07/28 03:34:43 DEBUG : rclone: Version "v1.55.1" starting with parameters ["./rclone" "-vvv" "serve" "http" "--addr" ":9097" "./"]
2021/07/28 03:34:43 DEBUG : Creating backend with remote "./"
2021/07/28 03:34:43 DEBUG : fs cache: renaming cache item "./" to be canonical "/home/ubuntu/rclone-v1.55.1-linux-arm64"
2021/07/28 03:34:43 INFO : Local file system at /home/ubuntu/rclone-v1.55.1-linux-arm64: poll-interval is not supported by this remote
2021/07/28 03:34:43 NOTICE: Local file system at /home/ubuntu/rclone-v1.55.1-linux-arm64: Serving on http://[::]:9097/
^C
I opened Possible issue with serve http on v1.56 - #2 by SimpleBobster because I hadn't seen this. It's got the full form filled out there.