Somewhere in Yiimp we are not converting from Bitcoin compatible nbits (target) to Siacoin compatible nbits.
Thus miners are being instructed to target much lower when they submit.

We believe they worked in testnet because it was shooting fish in a barrel. Almost any hash met target at a difficulty in the single digits.
Until we resolve the issue using Yiimp or another stratum we will be temporarily secured by GPU using stock blake2b without converting to Siacoin target format.

In the meantime, WildRig Multi 0.17.5 now has beta support for blake2b-btcc
https://bitcointalk.org/index.php?topic=5023676.msg51427516#msg51427516