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Hey! I've had this bug happen twice now (or something quite similar).
First time with a ward just like yours. Mine was in the bottom-most bush on blue side. Except it gave me vision of part of the bush, but not the top part. Champs would go in and not be visible until they got closer to the ward. Almost like there are two "sections" of the bush and the ward was only giving vision of one of them.
Second time didn't happen with a ward, but with my own champion's vision. I charged into the bottom part of the bottom bush (blue side) to give vision to my adc, but their Sona never appeared until I got quite a ways into the bush (too late by then of course, and I took "tons of damage" for my troubles).