Tested versions

Reproducible in: 4.5.1 stable [Linux Desktop], 4.5 stable [Linux and Windows Desktop], and 4.5 stable [Web Editor]
Not tested: Versions prior to 4.4.1 (But memory serves Line Separation worked without issues for me since messing with Themes. And I started using Godot at version 3)
Not reproducible: 4.4.1 stable [Linux Desktop]

System information

Linux Mint+Web Browser, Godot 4.5 (stable) - Vulkan (forward)/Compatibility on Web Browser - AMD 6900 XT dedicated GPU

Issue description

At first I updated from 4.4.1 stable which has working Line Separation on Themes to 4.5 stable. I noticed my game no longer had this property working. This was on desktop Linux. I tried to set it on Theme Override as well with no results. Lines were not any less or more separated when value was adjusted.

I then created a new project on the browser to rule out it just being a Linux thing or my own project converting wonky. The issue persisted even on a brand new project. This behavior is observed in both the Editor and in runtime for the game.

Steps to reproduce

Create or open an existing project -> Create a Node2d or Control node (Issue happens as child of both) -> Create RichTextLabel as child of Node2d or Control node -> Create a Theme, or use Theme Override (Issue happens with both values) -> Set "Line Separation" to any value other than default (0).

Last step, write any two lines of text into the "Text" field of the RichTextLabel. Ctrl+S to save. You will see in Editor the spacing doesn't change like in 4.4.1. Same happens if you go into runtime.

Minimal reproduction project (MRP)

line-separation-example.zip