That was a really sad time honestly. QCOMM went from leading the pack to basically using reference designs (which they still arrogantly brand as Kryo despite essentially being a tweak of a reference design.
It all happened because Apple came out with the first 64-bit design, and QCOMM wasn't ready. Rather than deliver 32-bit for 1 more year, they used an off the shelf ARM 64-bit design (A57) in their SoC called the Snapdragon 810, and boy was it terrible.
From what I gathered, they made at least _some_ risky architecture choices in their custom architecture that turned out not to be sustainable over the next generations. Also note that their Cortex core is indeed customized to a significant extent.
It all happened because Apple came out with the first 64-bit design, and QCOMM wasn't ready. Rather than deliver 32-bit for 1 more year, they used an off the shelf ARM 64-bit design (A57) in their SoC called the Snapdragon 810, and boy was it terrible.