It’s definitely a loose end. I was going to write that she disowned her social status and family, but than again she did write to her mother to tell her she was with child. To be fair is Talisa Maegyr why more invented than Jeyne Westerling to any other fictional character in ASOIAF and GoT?

She wrote to her mother to give her an excuse to tell Robb she was pregnant, to make her death that more horrific. 

As to why Talisa is more invented, it’s more that Talisa exists only to die. Jeyne has a specific backstory that is a significant part of the Red Wedding – she’s a Westerling, which creates a Romeo and Juliet thing b/c she’s a Lannister bannerwoman, her ancient but poor house maximally pisses off Walder Frey, her mother being Sybell Spicer the granddaughter of Maggy the Frog is linked to Tywin’s plotting to trap Robb at the Twins – and significantly, she survives the Red Wedding. There is more to her story than dying at Robb’s side.