I've always had the same mastery level on both hull and turret, as I've always use the same combo. Yesterday I reached mastery level 55 on both my hull and turret, but hull mastery level was not increased and remained at 54.
I have 305482 XP on Shaft #55, and 305351 XP on Viking #54 (correct mastery should be Viking #55).
Even when mastery was not increased, progress bar for hull was reset. Progress bar shows 451/11100 for Shaft, and 320/11100 for Viking.
Customer support service by UserEcho