The microcontroller-based OpenSprinkler runs a software TCP/IP stack on a 16MHz microcontroller, so it has limited capability to handle network requests. On top of that, it needs to perform certain tasks, such as pinging the router periodically to make sure it’s still connected, and perform NTP sync, and check weather. While it’s doing these tasks, it won’t be able to respond to pings. I am not sure how frequently you are pinging the controller, but frequent pinging will obviously flood the controller causing it to stop responding. In the end this is not a computer running a Linux system, it’s a small embedded system.