[Solved]-Same-size flexboxes are broken with Highcharts-Highcharts

As suggested here, you should add flex-basis: 0 and overflow:hidden to the chart containers. Note that if you have several levels of flexboxes one into each other, you should add this at every level.

#chart1 {
  background: lightcyan;
  flex-grow: 1;
  flex-basis: 0;
  overflow: hidden;
}

#chart2 {
  background: lightyellow;
  flex-grow: 1;
  flex-basis: 0;
  overflow: hidden;
}