Error, warning, weak-warning and typo underlines never showed on macOS, nor did underline effects of syntax tokens from a colour scheme: WebKit rejects a text-decoration shorthand with a style or colour (underline wavy <colour>), so the declaration fell back to none.
The editor's diagnostic and token styles also set -webkit-text-decoration, which WebKit takes the full value through; the red wavy underlines of the Problems, Build, Project and Database views use the longhand properties.