Maybe she was a child as a result of an affair?

All I can find about her mother is that the mother "was lost" during the Scourge attack on Lordaeron. So, maybe she's not confirmed dead. I mean, for all we know, the mother is also still alive and Taelia may (presuming) only have the word of letters written to her by Bolvar that she perished to the Scourge. And, as a result, she (and perhaps by extension, Cyrus) may have probably never made an attempt to confirm what Bolvar may have told them.

I mean, it kind of makes sense considering Bolvar, as the Lich King, didn't seem to use the opportunity with all the control over the Scourge to maybe, y'know, find his wife or anything...