yocto - Why is if statement always false?

I set a debug flag in my local.conf

IS_DEBUG_BUILD = "1"

but a if condition in a Yocto recipe outputs False all the time when I run bitbake -C compile my-recipe

do_install:append() {
    bbwarn ${@d.getVar("IS_DEBUG_BUILD")}
    if [ ${@d.getVar("IS_DEBUG_BUILD")} == '1' ]; then
        bbwarn "True"
    else
        bbwarn "False"
    fi
}

The line bbwarn ${@d.getVar("IS_DEBUG_BUILD")} returns the correct values (0 and 1).

What is wrong here?

Please try this code:

    IS_DEBUG_BUILD_VALUE=${@d.getVar("IS_DEBUG_BUILD")}
    
    if [ "$IS_DEBUG_BUILD_VALUE" = "1" ]; then
        bbwarn "True"
    else
        bbwarn "False"
    fi

The difference is in " " (double quotes) or ' ' (single quotes). They are used for respectively string or char type literals.
In your sample code, you first set the IS_DEBUG_BUILD to "1" (string) and then checked with == but you compared it to '1' (char). String and char are never the same.