Finally!
I understood it was due to HF19, but Poloniex never confirmed not denied this... Their support and public announcements are terrible! Poloniex just need to get this one thing right.
I used Changelly in the meantime, but it does not let me set limits.
What the heck!? Right??? I haven't heard of Changelly. I'll have to look into it. Poloniex needs to be more open about this stuff.