Something you might be overlooking is the difference in defensive attention. Last year, Lydon was cast in a complimentary role and thrived not being the focal point of defensive attention. The offense went through Gbinije, Cooney, and Richardson primarily last year, which enabled Lydon to camp out at the three point line and spot up.
As more of a main cog this year, he's getting more concerted defensive focus, and hasn't been as efficient.
Very similar to what happened with CJ Fair -- who like Lydon, was the ultimate complimentary offensive threat, but struggled as the main guy. Fair had an incredible season in terms of advanced metrics in 2013, when the offense ran through MCW and Triche, but those percentages plummeted the following year when he was the main guy. It wasn't a knock on his talent, it just highlighted that the role he was best suited for was complimentary scorer. Lydon's the same way.