Open Source is a mod to allow the creation of source blocks for all fluids when 2 sources are in a 2x2, creating an infinite water source, like how water acts in vanilla Minecraft.
This behavior can be disabled in the mod configuration, which is found in the following locations:
world/serverconfig/open-source-config.tomlAnd, if you haven't guessed, the name is a pun. The mod is not open source after all :)