I hope you bought more when you saw MML at this price (that's what I did)!
As for why that happened, I wouldn't get to concerned about MML being slow to respond or mispriced presently, there is a lot of uncertainty and negativity at the moment and low volumes probably haven't helped.
Ultimately the fundamentals will drive the share price in the long run. Just need to have patience and conviction in your trade until we are given the next update.
In the short run, being out of the ASX300 will mean that MML will likely be off the radar of most traders and fund managers. So it won't be "super responsive" to gold movements or news flow until it's back on their radars (ie when its back in the ASX300 and GDXJ).
This probably won't happen until the next quarterly/annual report (assuming they are positive) or until gold closes north of $1350.
Until then, just take advantage of the bargains and think of all those selling MML at <$0.64 yesterday when the gold price was above $1300+ like this: