Hard to imagine. A lot of times why these things are so hard for developers to fix are because of how edge-case they are. If this was happening on every game or in some consistent way, then it would be really easy to pinpoint. If we even had like 3 examples of this it would probably make a lot more sense.
As it is with the knowledge we have, it’s really hard to guess.
One extra clue of weirdness To notice though is that the units are being highlighted properly. So the targeting system thinks they’re possible targets but the part that’s failing is the validation before it lets you click them. So it thinks they’re possible targets but not valid ones and so it cancels the click. (Or it doesn't think you're clicking on them at all)