I think that in all the following cases the value of `$NF` should be the empty string as is output by gawk (see below).

{{{
# printf "a==123=" | awk -F '=+' '{print $NF}'
=
# printf "a===123=" | awk -F '=+' '{print $NF}'
3=
# printf "a====123=" | awk -F '=+' '{print $NF}'
23=
# printf "a=====123=" | awk -F '=+' '{print $NF}'
123=
}}}

This is the output from gawk:

{{{
$ printf "a==123=" | awk -F '=+' '{print $NF}'

$ printf "a===123=" | awk -F '=+' '{print $NF}'

$ printf "a====123=" | awk -F '=+' '{print $NF}'

$ printf "a=====123=" | awk -F '=+' '{print $NF}'

}}}