What went wrong
There is no expansion gap. Somewhere in that room the planks are butted tight to a wall, a jamb, a pipe collar or a hearth, and the floor has nowhere to go when it grows. So it went up instead, usually at a seam two rows from the sofa, usually in the first warm week of summer. It is a quiet failure: no water, no crack, no warning at all.
You will not see it from standing height. The gap lives under the baseboard, behind the door casing, inside the pipe collar, under the cabinet toe-kick and behind the transition strip. Each is a place where a plank end meets something fixed to the building, and none is visible once the trim is back on.
The annoying part is the delay. The floor went down in autumn, looked perfect for months, then buckled in June. Nothing was dropped on it. The installation was already wrong; the weather just made it visible.
Then people fix the symptom. They weight the ridge down with books, trim the plank at the seam, run caulk under the trim because the gap looks like a draught. That hides it until the next humid week, and some of it makes things worse.
The fix is unglamorous: restore the gap on every side, then refit the trim so it covers the gap without filling it. If a seam has already peaked, deal with that first, because a buckled seam is a symptom with its own repair.
Why it happened
A floating floor is not attached to the house. The planks are locked to each other, the surface sits on underlayment, and it slides a little as humidity changes. Free to slide only works if there is somewhere to go, which is why the carton prints a perimeter gap.
The number varies. A common figure for laminate is around 3/8 in (10 mm), and for LVP nearer 1/4 in (6 mm). Treat those as a guide. The carton is the authority, and if it disagrees with anything online, the carton wins.
The gap goes missing in the same places. Under baseboard, where the plank was tapped tight to the drywall. At door jambs, where the plank butts the casing instead of sliding under it, which is its own small job. Around pipes, where the hole was drilled to pipe size. Behind a hearth, where the floor was cut to the tile line. Under a cabinet toe-kick, where cabinets went in afterwards and were fixed through the planks. At a transition strip, where the track was screwed through the floor.
You can check most without lifting a board. Slide a coin under the shoe moulding; if it will not go in, the floor is tight. Then look at the casing.
Do not fill it with caulk, foam or grout. It is a movement joint, not a draught. Filling it makes a rigid block and the buckle returns. Cover it with trim: baseboard on the wall, shoe moulding above it, both clear of the planks.
When to stop and call someone
Stop if the floor is glued down, solid timber or nailed. Those behave differently, and a perimeter gap is not the repair. Stop too if the buckle returns within a week of cutting, if the planks feel soft at the edges, or if the floor is over a slab with no vapour barrier; that is water, not expansion. If an island or built-in sits on the floor, get advice before you cut, because moving one is a two-person job.