Trouble with custom date hierarchy

Hello,
I am trying to create a report with a custom date hierarchy: Year/Month/Week.
For my measures, I use the following definition (other fields omitted):

'claimObject.date_mission.y': {type: 'level', caption: 'Year', hierarchy: 'Mission Date', level: 'Year'},
'claimObject.date_mission.m': {type: 'level', caption: 'Month', hierarchy: 'Mission Date', level: 'Month', parent: 'Year'},
'claimObject.date_mission.w': {type: 'level', caption: 'Week', hierarchy: 'Mission Date', level: 'Week', parent: 'Month'}

then in slice (other fields omitted):

columns: [
  {
    uniqueName: 'Mission Date',
    levelName: 'Month',
    filter: {members: ['year.[2017]']}
  },
  {uniqueName: '[Measures]'}
],
drills: {
  columns: [
    {tuple: ['year.[2017].[january]']}
  ]
}

Everything works as expected except that the report is not opened with January drilled down. If I drill manually, it
works except that the produced report contains (same column repeated twice):

drills: {
  columns: [
   {tuple: ['year.[2017].[january]']},
   {tuple: ['year.[2017].[january]']}
  ]
}

If I click on the week drill down of February, it drills correctly and the report is now (as expected):

drills: {
  columns: [
   {tuple: ['year.[2017].[january]']},
   {tuple: ['year.[2017].[february]']}
  ]
}

Sadly, using this slice configuration to open a new Flexmonster does not perform any drill. I suspect I am misusing hierarchies.
What is the correct way of working with such a hierarchy ?