Julia 1.0.5: Using Pkg.add removes all [extras] dependencies from Manifest

General Usage
1 
julia> versioninfo()
Julia Version 1.0.5
Commit 3af96bcefc (2019-09-09 19:06 UTC)
Platform Info:
  OS: Linux (x86_64-pc-linux-gnu)
  CPU: Intel(R) Core(TM) i7-4770HQ CPU @ 2.20GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-6.0.0 (ORCJIT, haswell)

I create a new package:

(Modules-1.0) pkg> generate MyPackage
Generating project MyPackage:
    MyPackage/Project.toml
    MyPackage/src/MyPackage.jl

(Modules-1.0) pkg> activate MyPackage

I add Test as a dependency:

(MyPackage) pkg> add Test
 Resolving package versions...
  Updating `~/datascienceworkbench/lib/Modules-1.0/MyPackage/Project.toml`
  [8dfed614] + Test 
  Updating `~/datascienceworkbench/lib/Modules-1.0/MyPackage/Manifest.toml`
  [2a0f44e3] + Base64 
  [8ba89e20] + Distributed 
  [b77e0a4c] + InteractiveUtils 
  [8f399da3] + Libdl 
  [37e2e46d] + LinearAlgebra 
  [56ddb016] + Logging 
  [d6f4376e] + Markdown 
  [9a3f8284] + Random 
  [9e88b42a] + Serialization 
  [6462fe0b] + Sockets 
  [8dfed614] + Test

Next I quit and using my text editor move Test into [extras] to give a Project.toml like this:

name = "MyPackage"
uuid = "eb0aa806-0915-11eb-255b-a723c42b6e2c"
version = "0.1.0"

[deps]

[extras]
Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"

[targets]
test = ["Test"]

Now return to julia, and add another package dependency:

(Modules-1.0) pkg> activate MyPackage

(MyPackage) pkg> add JSON
  Updating registry at `~/.julia/registries/General`
  Updating git-repo `https://github.com/JuliaRegistries/General.git`
 Resolving package versions...
  Updating `~/datascienceworkbench/lib/Modules-1.0/MyPackage/Project.toml`
  [682c06a0] + JSON v0.21.1
  Updating `~/datascienceworkbench/lib/Modules-1.0/MyPackage/Manifest.toml`
  [682c06a0] + JSON v0.21.1
  [69de0a69] + Parsers v1.0.11
  [2a0f44e3] - Base64 
  [ade2ca70] + Dates 
  [8ba89e20] - Distributed 
  [b77e0a4c] - InteractiveUtils 
  [8f399da3] - Libdl 
  [37e2e46d] - LinearAlgebra 
  [56ddb016] - Logging 
  [d6f4376e] - Markdown 
  [a63ad114] + Mmap 
  [de0858da] + Printf 
  [9a3f8284] - Random 
  [9e88b42a] - Serialization 
  [6462fe0b] - Sockets 
  [8dfed614] - Test 
  [4ec0a83e] + Unicode

Instead of just adding JSON and its dependencies, it also removes Test and its dependencies from the Manifest.toml file.

How do I stop this from happening?