Why does my paper model's tail keep splitting at the seam after a day or two?

The gap opening overnight is classic memory pull. When you curl a panel and glue it under tension, the paper wants to spring back toward flat, and white glue doesn't have enough grab on cardstock to fight that, especially on a narrow tab. The glue isn't failing, the joint is just under constant load with a weak bond line.

What fixed it for me on a curled husk tail was twofold. First, I stopped relying on the tab alone and made the curl from one continuous piece where possible, so the seam sits somewhere in the straight-ish middle of the curve instead of at the tightest point. Second, I gave the paper a head start. Before assembly I rolled the pieces around a marker or dowel a few times, both directions, until the curl held its shape on its own with barely any tension. If the paper alredy wants to be curled, the glue is only holding a seam, not fighting a spring. That alone made the biggest difference. On tabs, yeah, tighter curves need wider ones. On a gentle bend a 5mm tab is fine, but for something like a Spitz tail curl I'd go 8 to 10mm and clip tiny notches into the tab so it can bend without buckling the mating edge. Scoring the tab lightly also helps it follow the curve instead of lifting.

Thicker cardstock will make the curl fight you harder, so I wouldn't go up in weight for this part. If anything, a lighter stock just for the tail curls easier and holds a set better. Some people mist the paper very lightly with water before curling to relax the fibers, then let it dry in the curled shape. Works well but you have to test it on a scrap first since some stocks warp or show water spots. For glue, look for something that stays tacky or flexible, like a tacky craft glue or a thin coat of contact cement on both surfaces, let it dry to touch, then press together. Contact cement is great for this because the bond is rubbery and absorbs the flex instead of cracking. Regular PVA dries hard and brittle, which is exactly why it lets go on a stressed curve. Give the seam a few minutes of firm pinching while it sets and you should be done re-gluing weekly.

This explains so much about the little cone-shaped feet on my last build, they did the same slow-peel thing and I kept just smooshing more glue on them. Never thougt about the paper still 'wanting' to uncurl after it's glued. I'm gonna try rolling the pieces around a marker first next time. Does the rolling thing work on small pieces too, or only long ones like a tail?

One thing that helped on my last curved section was backing the seam from the inside with a narrow strip of thinner paper, glued across the joint after the main assembly sets. It spreads the stress over a wider area instead of letting the whole load sit on that one tab line, and since it is hidden you can be as sloppy as you like with it. I also leave the tail pinned around a form overnight with the glue curing under compression rather than tension, which seems to matter more than I expected.

Update: tried the rolling trick, worked the curl around a spray can overnight before gluing and the seam sat way more relaxed. Held fine for three days now. The backing strip idea makes sense too, but should I glue that strip on while the tail is still off the body, or wait untill the whole thing is assembled? Not sure I can reach inside once it's attached.