New brass nearly always stretches (and stretches the most) on its first firing.
Every new case I've handloaded for .222 Rem. .243 Win., .270 Win., 7x57 Mauser, .308 Win., .30-06, 7.65x53 Mauser, 8x57 Mauser rounds has got shorter when first fired. Same for factory or arsenal ammo in most of those cartridges. Their bodies were larger after firing, so the brass to do that had to come from someplace. Betcha a dollar it got drawn back from the shoulder and neck of the new case.
They all got longer after being full length resized. Then when those reloaded cases were fired again, they all got shorter.
I don't think any case headspacing on its shoulder gets longer when fired. Even my 30 caliber belted cases full length sized to headspace on their shoulder get shorter when fired.