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You'll need to make sure there are flat spots where the circular arc transitions into those T junctions. Onshape's sheet metal can't currently support doubly curved geometry or lofted transitions.

Turns out it is to do with the way Onshape determines what a THRU hole is - it uses some moderately complex logic as to whether a hole goes all the way through a part or not; and in this case I had a U shaped sheet metal piece so that in the folded state it decided they were not through holes because there were more surfaces of the part in the way, though not when flat patterned:

The workaround for now is to make the holes start from the inner face and terminate on the outer face.

I have submitted an improvement request for it to simply follow the design intent (ie. the termination setting) entered by the user:

I appreciate the programmers might've been trying to handle holes not created by the hole tool or something, but sometimes KISS is best - just let the user decide rather than trying to handle it via logic.

Here's a simple example with a bunch of holes defined with only a few hole features, but look at how complex the hole table is…

https://cad.onshape.com/documents/76b89c0e6dde8188dd4238ff/w/b10a0384451697ee355b7db3/e/6b77f25c0ad9b23b83ebae17?renderMode=0&rightPanel=holeTablePanel&uiState=66fcd12808682a02ab9824e3