Is it normal for a 1/4 scale print to warp at the base after painting?

The lifting is almost certainly related to differential expansion and weight stress rather than just the cure time. When you cast a large resin piece, especially one with thick sections like a heavy base, it takes a long time to stabilize internally. Even if you cured it perfectly, thermal expansion and contraction when it moves from a warmer workshop to a cooler room will cause minor, but measurable, shifting. The weight of that heavy slab, combined with those subtle movements, is what's stressing the bond at the edges. For a piece this substantial, relying on its own static placement on a shelf is a risky bet if you want it to look pristine for years. If it's already lifting, waiting for it to settle is pointless; the movement will just start again when the temperture shifts next week. You definitely need to secure the base down.

Don't just slap some quick craft glue on it. You need a serious adhesive. Since you are gluing plastic and what is likely a composite or stone-like mixture, look into a high strength two part epoxy or even a structural polymer adhesive designed for heavy outdoor or display pieces. Apply the adhesive to the contact points on the base and then adhere it to a stable, perfectly level surface. Use clamps, or even heavy weights, to keep the pressure on for a while as the adhesive sets up to prevent that corner from lifting further. In the future, when casting large environmental pieces, plan for them to be mounted. You can permanently adhere the entire base to a piece of solid wood or metal that sits on your shelf. This transfers the physical weight and stress away from the fragile resin structure itself. Even if you are mounting it to a piece of wood, you should still fill that gap under the roots or any visible lifting with a filler material to maintain a consistent profile. That way, the finished piece looks like a solid rock sitting on the shelf, not something that's merely resting on it.

One thing I'd check before blaming the resin itself is whether the base warped during post curing, because big flat slabs are notorious for this if they cure resting on an uneven surface or get propped up at a corner while still soft. I had a 1/4 scale wrecked ship base do exactly that a while back, sat proud in the middle, and re-flattening it involved warm water and a sheet of glass and a fair bit of patience.

Also worth checking the shelf with an actual level rather than your eye, since a bowed shelf will make any heavy print look like it's lifted when really the shelf has sagged. Is your shelf solid wood or one of those particleboard flat pack ones? The latter creep over time under a load like that.

Agree with mounting it to something rigid, that fixed the exact same problem for me. My 1/4 scale dragon base did the same corner lift after a few months, so I stuck it down to a piece of MDF cut just slightly smaller than the base footprint with two part epoxy, stacked a couple books on top overnight, and it hasn't shifted since. The board also hides the gap when viewed from standing height, which was my main worry. Weigh it though before you commit, because once that slab is fixed to a board you lose the easy option of moving her around the shelf.