@p command not functioning properly

'/gamemode a @p' command changes the gamemode of the second closest player instead of the closest player when executing it from a command block.

'/gamemode a @p[r=10]' shows 'no targets matched selector' when the first closest player is inside a 10 block radius and second closest player is outside of a 10 block radius. 

'/tag @p add <tag>' adds a tag to the second closest person instead of the closest player when executing from a command block. Same as '/gamemode a @p'.

The issue only occurs when executing the command from a command block and works properly when executing it from chat.