Stand-alone part extrudes, but not when added to the main part - Issue is back again

I am barely more than a novice. I am working on a simple form: When The internal supports sketch is extruded as a new part, it works great. When I change the extrusion to Add and merge with Part 1, It becomes non-manifold. I don't understand why, or how to fix it. This problem went away for part of a day, but now it is back. I have checked different browsers just to make sure, and I really need help.

https://cad.onshape.com/documents/ecb612f3f0ebf941e654c5dd/w/3916a6625a092ef17848e5e8/e/c34438349e6081f7a04e02f6?renderMode=0&uiState=67cc9de224fe9225e8750cb3

Can someone take a look and tell me what I messed up?

Thank you