D&D Classics - Single Instance of windows in FGU

I have an issue D&D Classics ruleset in FGU. When I open a window for like the fighter for example, then I open a second class like the Magic User, the fighter window closes and is replaced by the magic user. I can't have 2 classes open at the same time. This same thing happens with items, spells, and NPC's etc. The second window seems to replace the first instance of the first. In all of the other rulesets that I have used you can are able to have multiple class, item, npc, spells, etc, windows open at the same time. Is this a know bug or a feature? Please advise. Thank you.

I believe you need to hold down CTRL whilst clicking to open the window normally. (It might be ALT or SHIFT). There may also be an option in options but I’m not sure about that.

Much appreciated Zachaeus