I started a new game with the recent nightly (3820), with a Lothlorien archer.
After character creation I had the following weapon skills:
25 / 15 / 15 / 42 / 15

I played several training fights, got to level 3 by doing some fetch quests, never took part in any real combat, and I got my skills up to
41 / 15 / 19 / 78 / 15

I'm on the third day, guiding some refugees between two locations, arrive to my capital, go to the training grounds, check my characters, and my skills are suddenly
31 / 11 / 15 / 48 / 11

I did not get wounded (never took part in a real fight), I have no special traits on the traits screen, nothing.

I have saves from all the 3 cases where I listed my weapon proficiencies.