Steps to Reproduce:
1) Run 
/scoreboard objectives add killedbyplayer minecraft.killed_by:minecraft.player
/scoreboard objectives setdisplay sidebar killedbyplayer

2) Place a tower of 30 scaffolding blocks and summon a pig with a saddle on the top
3) Give yourself a drinkable potion of harming and a carrot on a stick
4) Begin climbing to the top of the tower and drink the potion near the top of the tower. Ride the pig off the tower with the carrot on the stick and observe the death message.

Expected result:
"<player> hit the ground too hard whilst trying to escape <player>"
and the counter to go up by 1 as this counts as the player killing and dying of fall damage on a vehicle normally produces "hit the ground too hard"

Observed result:
"<player> fell off scaffolding"
and the counter goes up by 1