This is from bug 1844870. See this testcase.
Firefox 115 puts all the content in one column. Google Chrome 117 & Safari 17.0 create many short columns, and the multicol container's width is way larger than the browser window width.
The only relevant spec I found is a note in https://drafts.csswg.org/css-multicol/#pseudo-algorithm
The used width U of the multi-column container can depend on the element’s contents, in which case it also depends on the computed values of the column-count and column-width properties. This specification does not define how U is calculated.