When trying to change Minecraft window title in 1.15.2, it does stay on default one.

randompatches-1.15.2-1.21.0.1.jar
forge-31.0.14

I've introduced another configuration option, window.titleWithActivity. It takes two arguments: the Minecraft version and the current activity. For example, in "Modpack - %2$s", the %2$s would be replaced by the current activity.

Ah, this is because Minecraft now changes the window title depending on context.