OK, figured it out. It's a Detail view (Master/Detail), and on an iPhone Simulated Metrics (5th panel), Size has to be set to Detail rather than Inferred. If Inferred, for iPhones it's covered by the Master, even though obviously on an iPhone it wouldn't be. Looks like a bug.
dcostin-dev
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Clean build folder didn't fix it.
Still there in Xcode 11.7
This seems to be intermittent. Try enough times and it goes through. You need to want it to go through, badly enough, perhaps.
Happening here too