This is a fun one to think about. I believe it is more than one thing. Heat on the brass and something else.
Is your COAL is 3.1 per book?
What is your barrel length?
Half a grain below book max and 100fps faster. QuickLoad says dangerous pressures at that speed but takes more powder to get there. Pfhhggg what does QL know?
Naw really, I don't have enough data to believe QL but it gives me 'relative' information. QL is 100fps slower than book. WUWT?
Tight neck? Have you measured the ID of the fired round neck? Will a bullet just slip fit into a fired neck?
Tight chamber? Overall could raise the pressure.
Tight head space? The 'hot' brass could be pushing back on the bolt.
Hot batch of IMR4350? I've heard of this but not encountered is AFIK.
If your barrel is 24" and a fast twist (1/9?) it could create more pressure but that much?
Or, since it releases after a short time, you are having some form of thermal interaction with one or more of the above. Maybe the chamber or head space is just tight enough that the hot case is pushing on the bolt but when it cools for 15 seconds as you say, it's fine.
Or like others suggest, the annealed brass is yielding more than worked brass. Combined with thermal expansion, it could hold the bolt until the brass cools a little.
If it were me:
Try full length sizing without annealing and see if it's still sticky but releases after 15 seconds. To me that would indicate heat not excessive yielding of an annealed shoulder. With that I would suspect a tight neck or chamber.