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

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

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

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