This has been happening for a bit now, maybe since the November update. I made no changes in the config file, but lanterns (normal, tuna, chinese) are not stacking in your inventory. They will still stack if you throw them on the ground and pile them into one stack. You can then pick them up fine in the new larger stack.

Maybe it has something to do with the items containing (or potentially being able to contain) fuel in them. It's been odd because I'll still get increased stack sizes from my loot table, but they just won't combine and stack within an inventory.

Make sure you are using 2.0.2.
Just tried stacking lanterns and it worked fine.
The newest version has had issues with weopons stacking as well.