I tested with UE v16.00.0.1032 (and v16.10 Beta II) a Find with option All Open Files enabled with the file tabs bar setting Single Line - drop down file list used for the dockable and and also the undockable bar. I opened 7 files, but only 6 displayed in the file tabs bar. So one was always only visible in the drop down list - later test also with 8 files, no difference. I could not see that Find Next ignores one file. But I saw the problem.
The find starts with the active file, it continues the find on the file with the tab right the active file, and so on. When most right file is reached, the next currently file with a currently hidden file tab becomes active. But the file tab of this file is not inserted on right side of the bar and all others moved left dropping out the currently most left one as everyone would expect. The behavior of this bar is that a file with a currently hidden file tab becomes active with its file tab inserted on left side of the bar moving all other file tabs to right and most right file tab disappears. The find command continues now correct on the new active file with the file tab not displayed before now on most left side of the bar. Now the problem starts. After all occurrences of the searched string are found in the active file, the find continues on next right file according to the file tabs bar and so on and therefore the find searches again in a file which was already searched for.
Please report this issue by email to IDM support.
Replace All in all open files worked fine in my next test using the same settings as for the find in all open files.