Dialogs open up maximised when parent rule is maximised

I’m on Plasma 6 but I had this same problem for years on Plasma 5 too. When I use Window Rules to add some properties to a program to Maximise horizontally and vertically on a particular virtual desktop it also affects dialogs. I’ve set “Window types” to “Normal Window” and, for example, “Window class (application)” is set to “thunderbird” in one case.

What tweaks do I need to apply to the Window Rule so that the parent window is maximised but dialogs open up at their default size?