Beers with roasted barley come out overcarbed

I've pretty much settled the protocols for mashing, sanitising, yeast maintenance, and bottle conditioning. Generally my beers come out decent. With one exception, anything that uses roasted barley.

If I use roasted barley, a ready beer is usually quite good in the first 1-2 weeks after usual period of bottle conditioning (7-12 days). When age of that beer approaches 1.5 months, bottles turn to gushers. I don't taste any off-flavours that would hint of infection, however, overall beer tastes more acrid/sharp/bitter than when it was fresh.

Here's what I tried to fix it: 1. Use fresh yeast, or use a different strain. So far I tried WY London Ale (few varieties), WY West Yorkshire, WY San Diego Super, Mangrove Jacks M44. 2. Mash longer. Temperatures tried were int the range 66-69C. 3. Ferment longer. Raise the yeast from sediment by rocking the fermenter. Raise fermentation temperature after 1-2 weeks, to help the yeast to finish. 4. Mash without roasted barley, and add a "cold brew" of roasted barley at the end of boil. 5. Adding 10-20% less priming sugar.

None of those fixes actually helped. Oh, and btw, I did't dry hop any, so hop creep is not a factor, too.

I start to think that roasted barley may provide sugars that are fermentable on a few months timeline, rather than few weeks, but was so far unable to read anything about it.

Any commends on what else I might be missing?