Reverting default beam endings

To typeset beams grouped 3-4-3-2 in 12/8 it is necessary first to override the default beam endings in 12/8, and then to set up the new beaming endings:

\version "2.24"

\relative c'' {
  \time 12/8

  % Default beaming
  a8 a a a a a a a a a a a

  % Set new values for beam endings
  \set Score.beatStructure = 3,4,3,2
  a8 a a a a a a a a a a a
}