Just mentioning the required number of builders is a spoiler.

I have 2 that Should Be Master Builders - the "skill line" is almost to the end but they are still adept? Go figure - I have had these two building anything and everything in sight and they stay at adept.....frustrating LOL

The transition from adept to master isn't easily discerned because the skill bar "grows" in "chunks" as a villager gains skill. The chunk that they appear to be stuck on is where that transition from adept to master takes place - somewhere in the middle. It's not a bug, although Arthur agrees that it can be confusing.