Water column loses volume without increase in density with WCSPH

[efaulhaber]

I noticed that the initialized water column in dam break examples with WCSPH collapses without an increase in density, as if the particles are initialized too far apart for the corresponding density.
I reproduced that behavior with a simple "fluid at rest" example in 2D.
The fluid is initialized like this. I added the gray box for reference.

When running the simulation, the fluid collapses and comes to rest at about 80% of the initial volume, but without a corresponding increase in density:

I also noticed that I need a very small CFL number of 0.01 for stability. With CFL = 0.1, I get these little explosions in the fluid:

What is going on here? Is there something wrong with my setup?

{
	"Configuration":
	{
		"pause": true,
		"sim2D": true,
		"particleRadius": 0.005,
		"colorMapType": 1,
		"numberOfStepsPerRenderUpdate": 4,
		"density0": 1000,
		"simulationMethod": 0,
		"gravitation": [0,-9.81,0],
		"cflMethod": 1,
		"cflFactor": 0.01,
		"cflMaxTimeStepSize": 0.005,
		"maxIterations": 100,
		"maxError": 0.1,
		"maxIterationsV": 100,
		"maxErrorV": 0.1,
		"stiffness": 100000,
		"exponent": 7,
		"velocityUpdateMethod": 0,
		"enableDivergenceSolver": true,
		"boundaryHandlingMethod": 0,
		"enableVTKExport": true,
		"dataExportFPS": 50,
		"enableRigidBodyVTKExport": true,
		"particleAttributes": "velocity;density"
	},
	"Materials": [
		{
			"id": "Fluid",
			"surfaceTension": 0.0,
			"surfaceTensionMethod": 0,
			"viscosity": 0.02,
			"viscosityMethod": 1,
			"vorticityMethod": 0,
			"vorticity": 0.1,
			"viscosityOmega": 0.0,
			"inertiaInverse": 0.5,
			"colorMapType": 1
		}
	],
	"RigidBodies": [
		{
			"geometryFile": "../models/UnitBox.obj",
			"translation": [1,1.0,0],
			"rotationAxis": [1, 0, 0],
			"rotationAngle": 0,
			"scale": [2, 2, 1],
			"color": [0.1, 0.4, 0.6, 1.0],
			"isDynamic": false,
			"isWall": true,
			"mapInvert": true,
			"mapThickness": 0.0,
			"mapResolution": [30,30,20]
		}
	],
	"FluidBlocks": [
		{
			"denseMode": 0,
			"start": [0, 0.0, -1],
			"end": [1.99, 1.0, 1],
			"translation": [0.005, 0.005, 0.0],
			"scale": [1, 1, 1]
		}
	]
}