Jldoctest blocks in julia docstrings is not evaluated

Would someone please have a look at the following issue?

https://stackoverflow.com/questions/57461225/jldoctest-blocks-in-julia-docstrings-included-in-documentation-but-tests-not-run

I also have the exactly same problem but could not find any similar issue reporting in the Julia Discourse.

Simply put, as I understand it, the jldoctest block in the doc string must be evaluated during the course of makedocs() run. but which seems not (seems the jldoctest block is silently ignored).

if I place the jldoctest block in index.md file, then I observed that jldoctest block mentioned in the index.md was evaluated.

For me, the configurations are (in Julia 1.3.0):
(sorry, I don’t know how to escape ```)

The index.md:

# MyPkg.jl

```@meta
CurrentModule = MyPkg

```@contents

## Functions

```@docs
func_sum


## Index

```@index

and in MyPkg.jl:

module MyPkg

"""

```jldoctest
julia> MyPkg.func_sum(1,2)
4

"""
function func_sum(x, y)
    x + y
end

end # module

How could I fix it?