First, thank you for the very prompt response (and the wonderful canjs project !). With your fork everthing work well ! I tried to reproduce these issues with my package.json, webpack config.... no effect, everthing work well...
Finally I delete my "node-modules" directory and then re-run an npm install for my project => No more issues
Next time, I will try that if a stange behaviour appears...
For the choice "steal vs webpack" is not my main priority because I migrate my code from requirejs/canjs 2 using strict ES6 syntax without extra webpack loader prefix :
So I think I will switch to steal.js without code migration. I' like webpack loaders for js / css / sass / stache / svg (svg to font). I lose stache compilation on build (can-complile)...
Next step : mocha integration for test (webpack + babel + canjs + mocha...)
I will try steal.js after for the same project to compare these 2 solutions and validate a final choice.