It has something to do with the fact that riven's skills move her and poppy's skill moves people as well. Poppy doesn't lock people like nidalee into place, so it's a riven-related bug.
lee sin can permasilence her by doing something similar.
they hotfixed a tristana bug that silenced her skills a while back, so I don't think it's a matter of not knowing how to fix it. the first step would be to fix the bug that silences her like that, then work on any other bugs that crop up from poppy's displacement.
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maybe, lol