Well that's disappointing. If you can't rely on 
tblAssets.Version then you'll have to fall back to doing it yourself as you were originally trying to do. Unfortunately, that means that you'll need to modify your report every time a new version is released in order to keep it current.
Using the list from the latest Patch Tuesday vulnerability report:
Select Top 1000000
  Case
    When tsysOS.OScode Like '10.0.10240%' Then '1507'
    When tsysOS.OScode Like '10.0.10586%' Then '1511'
    When tsysOS.OScode Like '10.0.14393%' Then '1607'
    When tsysOS.OScode Like '10.0.15063%' Then '1703'
    When tsysOS.OScode Like '10.0.16299%' Then '1709'
    When tsysOS.OScode Like '10.0.17134%' Then '1803'
    When tsysOS.OScode Like '10.0.17763%' Then '1809'
    When tsysOS.OScode Like '10.0.18362%' Then '1903'
    When tsysOS.OScode Like '10.0.18363%' Then '1909'
    When tsysOS.OScode Like '10.0.19041%' Then '2004'
    When tsysOS.OScode Like '10.0.19042%' Then '20H2'
    When tsysOS.OScode Like '10.0.19043%' Then '21H2'
    ELSE '?'
  End As Version,
  Count(*) As VersionCount
From
  tblAssets
  Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
  Inner Join tsysOS On tblAssets.OScode = tsysOS.OScode
Where
  tsysOS.OSname = 'Win 10'
  And tblAssetCustom.State = 1
Group By
  Case
    When tsysOS.OScode Like '10.0.10240%' Then '1507'
    When tsysOS.OScode Like '10.0.10586%' Then '1511'
    When tsysOS.OScode Like '10.0.14393%' Then '1607'
    When tsysOS.OScode Like '10.0.15063%' Then '1703'
    When tsysOS.OScode Like '10.0.16299%' Then '1709'
    When tsysOS.OScode Like '10.0.17134%' Then '1803'
    When tsysOS.OScode Like '10.0.17763%' Then '1809'
    When tsysOS.OScode Like '10.0.18362%' Then '1903'
    When tsysOS.OScode Like '10.0.18363%' Then '1909'
    When tsysOS.OScode Like '10.0.19041%' Then '2004'
    When tsysOS.OScode Like '10.0.19042%' Then '20H2'
    When tsysOS.OScode Like '10.0.19043%' Then '21H2'
    ELSE '?'
  End,
  tsysOS.OSname
Order By
 [Version]