- The redis-server process is not crashing and being re-started?
- The redis-server process is saving its data to disk at regular intervals, with no errors reported in the log file?
The situation you describe can very easily happen if the redis-server process consumes all the server's memory, causing a crash, yet it has not been configured to successfully save its data to disk. If the data is not saved, Redis can't restore it after a crash. If you didn't happen to notice the crash and re-start, it will look exactly like Redis is unexpectedly erasing its data.
This is not the only possible way to encounter the symptoms you describe, but it has happened this way in the past.