Posted this in response to a bug report on the subject:
Let’s say you have a weapon with 8 blessings pre patch. Let’s say you had a tier 3 for four of those and a tier 4 for four of those. Let’s say the weapon got some new blessings for a new total of 10.
How it works is that each of your current blessings can contribute at most “1 point” towards fulling an increment of the new XP mastery track, and there is one increment for each total blessing (so 10 increments with 10 blessings).
A tier 3 or 4 blessing provides about 75% of an increment. Meaning .75 * 8 = 6 full increments. But with 10 increments now you’re at 60% of the mastery despite having so many tier 3 and 4 blessings. 60% of the mastery is about mastery level 12. Well below the max of 20.
This isn’t a bug. It’s nefarious design decisions. People heard of you have most/all of the tier 3 or 4 blessings you’d max the mastery. But there are half a dozen subtle ways that notion was eroded in practice to leave you at 60% or less.