Do you observe this issue on a real iPhone running iOS 26 beta?
On simulators running iOS 26 beta or iPadOS 26 beta (under Xcode 26 beta 1-3), I observe the two following issues:
When the first segment is selected and I select interactively a segment other than the first segment, the newly selected segment is hidden behind an opaque selection marker. This resembles the OPs observation in this thread numbered 789898.
When another segment than the first is selected and I select interactively another segment than the first, the first segment is selected instead. This corresponds to an issue reported in another thread numbered 791988.
On a real device, however, the segmented control works as expected. I have tested this with the same code on an iPad Air 13" M2 running iPadOS 26 beta 3 (and with the previous betas). The segmented control also works correctly even on simulators when I select the segment programmatically.
I currently cannot install iOS 26 beta on a real iPhone device to test this issue. Can anybody test UISegmentedControl on a real iPhone running iOS 26 beta, and thus verify if this is just a simulator issue?