Why doesn't flexbox column height align work?

Question

css

The layout has a header__main-row block (two green columns).
In theory, inside it, the columns should be aligned in height (I tried to reproduce it in a pepperbox - it works ), but for some reason this does not happen there (if you add text).
Tell me, please, the reason.

Answer 1

.header__main-row {
align-items: stretch;
}