The Dispensation of the Fullness of Times doesn't mean that the dispensation will begin with all knowledge and understanding - complete and whole. The early development of doctrine and theology proves that beyond any doubt for anyone who stops to consider it with any objectivity. New converts brought their existing biases and apostate beliefs with them into the waters of baptism, since there wasn't an established theology for decades after the Church itself was restored. Frankly, I believe it's only been in the last few decades that a correlated consensus has begun to emerge - that we have begun to see the Fullness of the Restoration.
Why would someone assume that a church that grew and changed and evolved and never accepted a closed canon or cessation of revelation would suddenly or even gradually stop growing and changing and evolving - and become closed? The name of the dispensation simply implies that a fullness will be reached within the dispensation (before its end) - probably the day before the Second Coming, which will end this dispensation.