I didn’t change skill training speed from the default value. I just realized that I completely failed to mention that this character had the Fast Learner trait, though. I’ve used that trait dozens of times before and never seen numbers like this however; I debugged another test case, on 7425 this time, and every duffel bag was yielding 19-24% on Tailoring level 2. The instant level-up might have been a fluke, but those numbers are still quite high, much more so than they’ve ever been in the past with Fast Learner.
Edit: More examples. On 7426 now. Built a Self-Bow and Fire-Hardened Wooden Arrows. Fired nine arrows at a Fat Zombie using “Take Aim”, and Archery and Marksmanship leveled up from Level 0 at 0% to Level 1 at 34%. It’s never happened this fast before.