angular-cli: 6rc.X --stats-json or sourceMap blocking at 95% when optimisation&buildOptimizer are set to true

Versions

Angular CLI: 6.0.0-rc.9 
Node: 10.0.0
OS: win32 x64
Angular: 6.0.0-rc.6
... common, compiler, compiler-cli, core, forms, http
... language-service, platform-browser, platform-browser-dynamic
... router

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.5.12
@angular-devkit/build-angular     0.5.12
@angular-devkit/build-optimizer   0.5.12
@angular-devkit/core              0.5.12
@angular-devkit/schematics        0.5.12
@angular/cli                      6.0.0-rc.9
@ngtools/webpack                  6.0.0-rc.10
@schematics/angular               0.5.12
@schematics/update                0.5.12
rxjs                              6.0.0
typescript                        2.7.2
webpack                           4.6.0

Repro steps

Build with this configuration :

 "optimization": true,
              "outputHashing": "none",
              "sourceMap": true,
              "extractCss": true,
              "namedChunks": false,
              "aot": true,
              "extractLicenses": true,
              "vendorChunk": true,
              "buildOptimizer": true,

Observed behavior

Blocking at 95% but folder and map exist on filesystem, cli never finish 95% step. (it works if disabled optimisation option) 95% emitting index-html-webpack-plugin

Desired behavior

Finish generation build with sourcemap when optimisation and buildOptimizer are enabled in configuration

update: same behavior in v6 release of angular&cli