Discovered an irritating bug with canvas hierarchy buttons

First of all, I've got my canvas buttons all working properly, so there's no functional issues as such, but there is a really irritating bug in Unity Editor:

If I have a bunch of buttons inside the Canvas and elect to rearrange those buttons out of preference - just to put them in the order I'd like them to be - it stops the button's OnClick function from working. Nothing I do can get them working again, unless I re-drag the button back to its original placing in the list inside the canvas.

So after i make all the buttons I want in my scene, they have to stay in that exact position under the Canvas forever, if I want them to stay working.

But Unity Editor lets you drag them around to rearrange them, yet they stop working if you do... so is this a bug?