What does it mean if a girl rarely mentions her boyfriend?

I'm friends with a girl who knows I have strong feelings for her, and there's an attraction between us (well, from my POV).

Our friendship has been up and down because of my emotional weakness about her being in a relationship already, but things are looking better now as I've accepted that she and I can only ever be friends (not only because she has a boyfriend, but because our life paths are very different).

But the strange thing is that she rarely mentions her boyfriend ... I've asked her once about him and she looked uncomfortable talking about him, so I don't bring the subject up anymore.

I have many female friends and they all talk about their boyfriends and their lives with them, so this is something strange and new to me.

Ladies, please help me understand what this means? I'd like to know more about her, but not hearing about her life with her boyfriend probably cuts 50% of how much I'll ever know about her.
What does it mean if a girl rarely mentions her boyfriend?
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