When you have jobs with high parallelism, the URL for a REST request can get very long. When the URL is longer than 4096 bytes, the REST API will return error
Failure: 404 Not Found
This can easily be seen in the Web UI, when Flink queries for the watermark using the REST API:
The request will fail with more than 170 subtasks and the watermark will not be displayed in the Web UI.