Ok I’ve noticed one extra thing which is definitely narrowing things down.

    After the reload occurs I notice my cursor is waiting for me to select a target item for the 

    /use Deadly Poison

    line of the macro, and the macro button itself is highlighted.

    Seems like 2nd line of macro

    /use [btn:1] 16; [btn:2] 16

    is causing issue.

    So removed that line and I get no reloads.

    Also no other addon is causing the issue, confirmed by process of elimination.