Brian,
I'm glad you found the problem! I don't know enough to comment on whether ESR matters, but you should be able to gather some empirical data by observing the actual time-to-stabilization for both crystals during wake up.
You can toggle a GPIO around the call LL_PowerOnReq( wakeForRF ) in hal_sleep.c and observe it with a scope or LA. I believe in that function is where the sleep driver spins waiting for the clock to stabilize in the 1.3 blestack.
-Tyler