Inconsistency in radio button tab indices

I've got several sets of radio buttons: Some inside a top-level groupbox, some inside lower-level groupboxes, and some on the form. All of them have their tab indices set appropriately and the TabStop property set to true. I want the user to be able to tab through every single radio button (not just the first in the group). The bottom-level groupboxes do this correctly, but the top-level groupboxes and the radio buttons directly on the form don't. The screencap shows which are which.



Is there a property of either the container or the radio buttons that governs whether they're considered a single control or multiple controls for tab index purposes? If not, why else would some of the controls, but not others, behave properly?

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