I left a solo world in my house (in-game), and when I re-entered it later I was at least 64 blocks above ground, and in the middle of nowhere, as far as I could tell.  I promptly fell to my death.

  It hasn't happened since, but now I constantly carry a potion of slow falling on me just in case.

Does anyone know:

a) Why it happened / how it occurred

b) How to avoid it

c) If they have experienced anything similar

I should note that this occurred on the 12th December 2018, so the day after the 1.9 (Pillage and Village) first update occurred.  I had spawned safely after the update until then, so I do not know how relevant it is.

Thanks.