Command block is at the bottom of a chain, 6 blocks under a pressure plate. The first block sets a scoreboard for <player> if tag=!leader

This finds the closest player in the given radius, but fails to execute:

/execute @p[x=~,y=~6,z=~,r=1.5] ~ ~ ~ tag @p[x=~,y=~6,z=~,r=1.5] add leader

"Failed to execute command as <playername>"

This works fine:

/execute @p[x=~,y=~6,z=~] ~ ~ ~ tag @p[x=~,y=~6,z=~] add leader

The same execute command with radius filter works for scoreboard players add commands. This bug only seems to affect the tag command for some reason.