You can find some recent discussion of Barkskin and AC at https://forums.larian.com/ubbthreads.php?ubb=showflat&Number=839490#Post839490.

As far as I can tell, the Barkskin AC calculation is working correctly, but the tooltips are sometimes incorrect which doesn’t help with any potential confusion! Apparently this is a known issue that will be fixed with the full release.

"You may call it 'nonsense' if you like, but I've heard nonsense, compared with which that would be as sensible as a dictionary!"