Structural Layer Weakness Halfway Through

I have a model that’s 20 layers tall and halfway through it the support structure changes from whatever infill pattern (Grid for example) to horizontal lines from layer 10 to the end of the last layer 20.

I’m using Cura 3.2.21 with the PLA(Village Plastics) Material with the Standard Profile of .25mm layer height.
My printer is the Taz 6 with the DualExtruder v3 and the 2nd extruder turned off in the Machine Settings
Standard .5mm Nozzle that came with the DualExtruder v3

Pretty much the default settings of the profile (well, I can’t find any setting that fixes this issue)
Bottom Layer = 4
Top Layers = 4
Layer Thickness = 1mm
Wall Thickness = 1mm
Wall Line Count = 2
20% Infill Density
Infill Patter = Grid (all patterns do the same thing)

Attached are pictures at different layers to illustrate what I’m talking about.
Layer 20 - The final layer
Layer 16 - The first of 4 layers that make up the “Top Layer”
Layer 15 - The last layer before the last top 4 layers get added
Layer 10 - A new pattern emerges. Horizontal lines now join the 2 segments together and continues through to layer 20
Layer 9 - Layers 5 through 9 look the same and as expected
Layer 5 - The infill pattern begins
Layer 4 - Bottom Layer, looks as expected

I am the one that created the shapes and “Merged” them together in the CAD program (Solid Works).
It looks to me that the slicing software is changing the infill pattern for some unknown reason. I can’t figure out how to stop that. I would expect layers 5 through 15 too look exactly the same with the only difference being one more layer higher than the last.

Any ideas why this is happening?

I couldn’t get these images uploaded in numerical order, instead they fell in place in reverse order. Maybe because I named the numerical?

Could you post the model in .stl format? I would like to take a look at it to see if I can reproduce the slicing behavior you described. Also, random suggestion, but have you tried rotating the way the model sits on the build plate? Sometimes models will slice different depending on how they are rotated, maybe it is just the way it is sitting on the build platform that is tripping up Cura.

You have a slight vertical angle in relation to the other. They are slowly tilting away from each other. If you look you will see 2 layers on the outside becoming three layers, indicating a slowly widening distance between them.

Problem Resolved!

First I want to say Thank You two for your troubleshooting efforts!

Your comments made me look at my model closer and revealed the error. It turns out part A was not attached to part B like I thought it was. There turned out to be a .17mm gap between the two even though I had a constraint on the corners of the second drawing to be coincident with the edge of the first drawing. I deleted the constraint and added a new constraint that the line on part A be colinear with part B. I would then extrude the drawing through all the layers.

The gap was enough to mess up the slicing. The model is now slicing as expected, all the way through the middle layers from bottom to top.