Wow. This one is subtle. It's not a bug, exactly, but it sure as heck isn't not-a-bug either.
Prior to the state above I had selected a polygon and used the Explode editing tool. Although I had finished with the tool it was not deactivated. There was no visual indication in the attribute table panel that the edit tool was still active and filtering records non-visually.
Said another way: the table panel was still showing me all records, but the allowable set of records that could be operated on were filtered by the still-active Explode tool, which meant that the Select By Attributes tool only saw the subset left exposed by Explode.
The fix was to deactivate the editing tool by using the back button in the tool panel and then use Select by Attributes again.
How did I figure this out?
Later, when I toggled OFF the layer's editability a temporary notification appeared saying "Active Edit tool is filtering the selection. Deactivate tool to restore the selection". At this point I remembered seeing the notification 15 or so minutes earlier when I started editing.
So the bug here if there is one is that the notification is temporary. Or that it shows at the wrong time. Or that the table panel does not have an indicator of the filtered record status. Or all of the proceeding.
See Esri's page on "active tool is filtering the selection" for more info.