Cura "initial layer height" bug?

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Hello UM community.

 

Recently I have been stumbling upon some failed prints which were poorle adhered to the build plate.

On closer expection it appears as if there was under extrusion on the first layer only. Assuming this was not a hardware issue I dove a bit into cura and found the following interesting result:

I tried slicing a tube which requires a good surface finish on all its sides. at a 30 degree angle I figured I'd get decent sides while maintaining stability over the entire height due to supporting structue. The position:

Slicing the model solo, which virtually does not touch the ground. The initial layer height appears to be ignored by cura and set to the layer height 0.15:

 

When I add a flat model to the mix, suddenly the initial layer height is turned up to the intended value 0.3:

 

Now this in itself would not be much of a problem. However the Ultimaker itself will raise up to 0.3mm on the first layer regardless of the sliced layer thickness. The result is about 50% underextrusion in the first scenario .

 

I am quite sure that this is what is happening. I tested this by powering off the UM3 on the first layer and attempting to then manually place 0.1 0.15 0.2 0.25 and 0.3 mm layers under the nozzle. The 0.25 was almost free moving while the 0.3 got significant resistance. Increasing the material flow for the first layer by a factor 2 also solved the problem in the first scenario, further confirming my measurement and suspision.

 

Is anyone else able to reproduce this result? If so is this a bug in Cura or am I incorrectly understanding the slicer? Any thoughts on the matter and how to tackle it would be helpfull as well.Manually checking the first layer thickness and adjusting the Initial layer flow is not realy managable with the amount of people that use the UM3 through the que.