I noticed in the packet dump you sent me the packets are being tagged with a VLAN which changes the ethernet header, specifically the type. If I am not mistaken, the Arduino library is very limited and not able to handle the VLAN packets and is simply ignoring them.
I understand you have the default VLAN select however I think the mere existence of that tag and the change of the packet from type IP to type 802.1Q (0x8100) causing the packet to be ignored.
If you can, please remove the VLAN tagging from the port the OpenSprinkler is using and see if that resolves the issue.