Have you calibrated your thermometer? You could be mashing cooler than you think.
Thermometers are calibrated. I have 5 on hand
1. No, I wouldn't crash while you are dry hopping - it needs to be done @ ~ 65 - 70F to extract the best hop flavor and aroma..
2. Are your OGs consistently low too, or just FG ? If you regularly are a few points low on your OG, it would carry over to making your FG low too.
3. Yeast attenuation rates are an estimate at best - a lot of factors go into determining what your actual FG will be - grist, mash temp, yeast health, pitching rate.
4. What temp are you mashing at ? Since it seems to be an AG problem and not an extract one, I wonder about the accuracy of your thermometer. If your thermometer is not very accurate, you could easily be mashing several degrees cooler than you think. This would make a more highly fermentable wort and would give you a lower FG than you anticipated.
5. What yeast did you use? Going from 1.084 to 1.016 is 80% attenuation and would not be out of line for a number of yeasts.
I almost always hit my OG and I'm using 1056 for my yeast. I did a starter because the OG was high but that's not always the case. With this batch I was aiming for 153° but overshot to 162°. I quickly got it down to my target temp but lost a lot of heat during the mash. It got down to around 148° but I brought it back up with boiling water. Now that I think about it, it was kind of a roller coaster mash
I had the same trouble for a while. Are you batch sparging or fly sparging? I batch sparge and used to let my first runnings sit in the kettle while I sparged. I now turn on the flame right away and start heating my first runnings while I sparge/vorlauf/drain my second runnings. It fixed my over-attenuation problem and I get to a boil faster. Win-win!
Funny because I do that every time! Just out of curiosity, what impact would that have?