Not true for high enough levels of jump boost. I am not sure if is it a bug, but jump height grows exponentially with jump boost level unlike fall damage reduction, so if you give yourself or a mob very hight level of jump boost, not all fall damage will be prevented.
If you're playing minecraft with a lot of mods on slower PC, things like this are going to happen. Try allocating more memory to minecraft or using optifine. It might help.
Do not use relative coordinates in functions, which will be run by gameLoopfunction. If you use this gamerule, the function will be executed by the server, not by the executor of the gamerule command. And because the server has no coordinates, relative coordinates will not work. If you want to make this loop working, you have to use execute command.
execute @a ~ ~ ~ /setblock ~ ~-1 ~ minecraft:cobblestone
#say hi
This should work OK
Still a bug. Invalid resource pack should never cause things like this to happen.