I believe cutouts & transparency apply themselves to each side of the element, so you have one cutout that is correct and one that is rotated, giving you the weird effect.

If you did model that it wouldn't be too hard - use sloped glazing, set your mullions to be the T profile and leave the panel blank.

Here's a "workflow" I've used before:
Create a "Rendering" DO with 2 options, Render and CDs, with CDs set to primary. Use the Rendering DO to hold all the elements that are only required for visualization. In this case, your documentation ceiling goes in the CDs, and a Basic Ceiling goes into the Render option. Override the DO display in your rendering views. Tip: When creating the Basic Ceiling, create 2 of them - a strip at the perimeter for the wall angle (no cutouts), and another one for the balance of the grid (your rendering currently omits the wall angles).

Cellophane is right - materials are side specific, so you'd need to get to a view where you can see the top side of the ceiling, and rotate/align its model pattern to match.

Otherwise, you can go the sloped glazing route, and get the thickness of the grid.

I cant help with the answer, but just wanted to say that is an impressive render. Composition, framing, lighting, textures, realistic...well done.

What Chad said. Sloped glazing, t grid modeled, no panels. I model all of my lay in this way now. I love it.

Sent from my Phablet. Please excuse typos... and bad ideas.

Aaron Maller
Director
Parallax Team, Inc.

Thanks all very much for the replies and advice.

I realized the issue was the cut out applying differently to all faces of the ceiling element so I went along with making the open grid actual 3D elements to render, and put my ceiling elements into design options so I wouldn't disrupt our CDs (good tip jmk, thank you!)

I didn't go with the sloped glazing route because it was much faster for myself to export the ceiling boundary/patterns to a DWG, separate the layers by Ceilings 1 & 2, and then use that DWG to create structural framing elements with Dynamo. I was also concerned about how the mullions might handle the curves of the wall angle, and the corners that occur where curve/arc meets a straight line, which structural framing elements handle well with beam joins... I ought to experiment with that though to see how mullions would look in those conditions, and I'll probably go with the Sloped Glazing method for open grid ceilings in the future though.
(by the way, cellersick, when I read your comment on the wall angle being omitted... *hardest face palm ever* xD)

Here is the outcome (of the ceiling, not the final final rendering... still working on improving the light fixture family on the grid to emit light better):


And for anyone who is curious, the Dynamo script:


Thank you all again

+1 for posting the Dynamo script!

Structural Framing by Dynamo method is awesome, but i use Sloped Glazing because- in most cases- i want the ceiling tiles themselves, as well.

Amy, can i ask how you got the patterning into the floor on the render? What workflow did you use? Or is it light and reflections? Its great.