I have an issue where my dark beers(Milk Stout, Dark English Mild and Russian Imperial Stout) have become overcarbed and gush out of the bottle when I open them. The FG gravity on each of them always ended high after at least21 days of fermentation at around 65 degrees. I always justified this because I have mashed high and have a lot of non-fermentables added(oats, lactose, crystal) These are all All-Grain beers.
Milk Stout FG: 1.022
English Mild FG: 1.020
Russian Imperial Stout FG: 1.035(OG on this one was 1.110)
I don't think this is an infection issue because I have bottled other beers(Pale Ale, Cream Ale, IIPA) that have good carbonation using similar amounts of sugar(around 4 oz each). Why would these darker beers not fully ferment out and start fermenting again in the bottle? I don't have an oxygen system, are darker, fuller beers harder to oxygenate through the shaking method? Would this cause fermentation to pick back up? Any other reason?
Any suggestions/ideas why this would be happening would be great!