MEP Transitions have inconsistent top

[Marc_L_]

When placing MEP fabrication transitions, what it calls top isn't always actually the top. See snip below, both trans were placed in plan view, then each selected and changed to Flat Top in "Edit Part" menu. How does revit determine what is the top of the fitting, and how can i make it consistent?

[tawilson]

I've noticed this too and your post decided me to try something. If the duct or fitting the transition is attached to is drawn in section view the transition shows the sides as being top or bottom. If the duct was drawn in plan view it showed everything the way it's supposed to be. Not sure if this is why it always happens but it did do it in this once situation.

[Marc_L_]

Yeah, revit treats the view you place it in as 'Top', whether its a plan or section. My issue is i'm placing them all in plan, and sometimes top and bottom switch, so when i want flat on top fitting, i have to always double check that top is top, and not bottom

[craigjonnson]

Each connector has an XYZ coordinate, something as simple as placing a shoe and rotating it to suit the airflow can flip the connector orientation. Any fabrication fitting applied thereafter could be upsidedown. Our team are aware of this issue and always check the Flat on top option, then rotate the fitting to suit. 
There is more information here... REVIT FABRICATION ATTACHER ARROW – MICLOGIC (home.blog)

[Marc_L_]

That...both makes sense and is really dumb. However I'm not convinced that's the issue here, as I haven't rotated anything. I like the idea of the attacher you linked, will definitely check that out!

I would still like to better understand why this is happening though. Doing some more testing while typing this reply, the behaviour of transitions really has me shaking my head. If i attach the trans direct to the fan, then top is top. If i attach a different fitting to the fan, like a canvas or a straight, and then the trans, i get the behaviour as shown in the snip in the OP. However if i keep adding straights between the canvas and the trans, i got even more bizarre behaviour. With a canvas, a single straight and the trans on each side, i ended up with top being bottom for both trans, and then for each straight i would add between the canvas and the trans, top would flip for both of them. ie 2 straights has top as bottom, 3 straights had top as top, 4 straight had top as bottom

This is very bizarre to me and something seems very messed up about this implementation. If no one here can explain this i may report a bug.