I very much doubt if these signs have authorisation, so your hunch is right that they’ve been used incorrectly.
These double-headed arrows on dedicated lane ADSs date back to the old Circular 7/75, that was the forerunner of TSM Chapter 7 for layout and spacing rules. At Buchanan Computing we put them into an early version of our SignPlot software to implement this Circular, but then hid them away when TSM7 came out, showing that such arrows should no longer be used. (But they’re still there if you know where to look.)
I can see why some people are tempted to still use them to indicate a lane that has two route options ahead, but very rarely is it possible to show clearly which destinations apply to which arrow head. So a conventional map or stack type ADS, followed by a diagram 877 (with no destinations) to show which lane to use, accompanied by lane arrow markings is the best solution. Sometimes the lane arrows will suffice without the 877, depending upon how intuitive the lane guidance is, and whether any serious safety or capacity problems are caused by occasional vehicles being in the wrong lane.