Orange border encloses the outer multi-colum.
Blue border encloses the inner multi-column.
inner multi-column has column-fill: balance

aaa
bbb
ccc
ddd
eee
fff
ggg
hhh
iii
jjj
kkk
lll
  Expected rendering without blue and orange borders:

  aaa      eee    | iii      kkk
  bbb      fff    | jjj      lll
  ccc      ggg    |
  ddd      hhh    |
  

Other closely related test: Nested-multicol-fill-auto-M-Stenshorne.xht