The bug

Using the new 'function' feature of the 1.12 pre-releases, if you have a relative teleporting command continuously running via gameLoopFunction it can produce different results in Creative mode than it does in Survival or Adventure mode.

Having the function be called by a repeating command block works correctly.

How to reproduce

1. Download the attached function swing.mcfunction and place it in the following directory of your world folder

   data/functions/custom
   

2. Open the world
3. Stand on a block higher than the blocks around you
4. Use the following commands

   /scoreboard objectives add swing dummy
   /scoreboard players set @s swing 0
   /setblock ~10 ~-1 ~ sandstone
   /summon armor_stand ~5 ~3 ~ {NoGravity:1b,CustomName:"2"}
   

5. Set the function as gameLoopFunction

   /gamerule gameLoopFunction custom:swing
   

6. Switch to Survival mode

   /gamemode survival
   

7. Cast a fishing rod and try to hit the armor stand
   → You are teleported in a straight line
8. Reset the gameLoopFunction

   /gamerule gameLoopFunction -
   

9. Place a repeating command block with the following command and active it

   function custom:swing
   

10. Repeat steps 6 and 7
    → You properly "swing" now