Is it normal for composite boards to buckle this much near a stone wall?

The buckling you're describing on a twenty-year-old composite deck usally points to a failure in the ledger board or the joist hangers, especally since it's right against a stone wall. Water tends to get trapped between the deck framing and stone, leading to rot that eats away at the wood where the fasteners are supposed to grip. If those connections fail, the joists can drop or twist, forcing the decking boards to buckle upward as the frame shifts out of alignment.

Since it's low to the ground, you probly have poor ventilation down there. Over two decades, that moisture buildup will rot out a beam even if it's pressure-treated. The fact that it's getting worse quickly this summer suggests the structural integrity is compromised to the point where the weight of the deck itself is pulling the remaining fasteners out. You realy need to pull up a few of those composite boards in that corner to see the **joist-to-beam connections**. Don't assume it's just thermal expansion. While composite does move with heat, it doesn't typically lift and warp in a localized corner unless the underlying structure has stopped supporting it. If the tape measure shows a significant drop or a shift away from the stone wall, the joist has likely detached. You're looking at a situation where the rim joist or a primary support beam needs to be reinforced or replaced before that corner becomes completley detached from the house or the footings.

Once you get those boards off, check for 'soft' wood with a screwdriver. If it sinks in easily, the wood is shot. You'll likely need to jack up that section slightly, install a new treated joist alongside the rotted one—wich we call **sistering**—and ensure you use stainless steel or high-quality coated fasteners this time. Getting some airflow under there or adding a bit of flashing against that stone wall will help prevent the new wood from meeting the same fate.

I totally agree that moisture against the stone is the culprit here. One thing to check for when you pull those boards is the buildup of organic debris in that gap. Over twenty years, leaves and dirt pack into the space between the joists and the stone wall, creating a 'wet mulch' that never dries out. Even with pressure-treated wood, that constant contact with wet compost will eat trough the grain way faster than just humidity alone. Make sure you clear out any gunk before you start sistering those joists or it'll just happen again.

Once you get those boards up and see the damage, you’re definately going to need to replace those rusted-out fasteners. I’ve seen this exact stone-wall rot before and standard screws just won't cut it for the repair. You shoud grab a few boxes of Simpson Strong-Tie SDWS Timber screws. They have a double-barrier coating that acutally survives the moisture trap you've got going on there. They're way easier to drive into old pressure-treated beams than traditional lag bolts and they'll pull that shifting frame back into alignment much tighter.