Calling pmap within threads hangs

Hi, for some reason the code snippet below hangs. Does any one have an explanation as to why this hangs

using distributed
addprocs(2)
tasks = Vector{Task}(undef,4)
@sync for i in 1:4
       tasks[i] = Threads.@spawn begin
           pmap(x->x, WorkerPool(workers()), 1:10)
       end
end
fetch.(tasks)

Surprisingly if i first run pmap(x->x, WorkerPool(workers()), 1:10) before running the code above everything works as expected.

julia> versioninfo()
Julia Version 1.4.1
Commit 381693d3df* (2020-04-14 17:20 UTC)
Platform Info:
OS: Linux (x86_64-pc-linux-gnu)
CPU: Intel(R) Core(TM) i3-5005U CPU @ 2.00GHz
WORD_SIZE: 64
LIBM: libopenlibm
LLVM: libLLVM-8.0.1 (ORCJIT, broadwell)
Environment:
JULIA_NUM_THREADS = 4

Start by reproducing the Julia 1.4.1 example using JULIA_NUM_THREADS=4, addprocs(2), Threads.@spawn, and pmap with WorkerPool(workers()). Compare it with the version that runs pmap once beforehand; done means identifying why the threaded calls hang and documenting or correcting the behavior with a reliable verification.