Execute selector issues

Having an issue where a selector on an 'execute as' command seems to be working inconsistently. I'm running the execute from a command block and it is set to run as an armor stand, and it then runs a custom function which contains 2 commands. The second command is simple, 'kill @s' it runs fine, it kills the selected armor stand, but the first command is a setblock command, which is told to place at ~ ~-1 ~, I expect this to set the block below the armor stand, but instead it sets it below the original command block, what's going on here and how do I fix it?