I never seen, heard or read anywhere that Lien was difficult to work with, difficult as a person or had any drug problems or other issues. On the contrary, all the cast members of Voyager seem to have only positive comments about her.
if she had been a "pain in the a**" or if she had had some problems, then it would have been out long before she was dumped or at least got out when she was dumped.
If you look a a series like "Beverly Hills 90210", it was common knowledge that Shannen Doherrty was a "pain in the a**" for both co-workers and producers, Charlie Sheen's behavior was well-known before he was dumped from "Two and a half men" and Gary Dourdan's problems became common knowledge when he was dumped from "CSI".
Lien may have been a shy, unsecure and introvert person back then and being fired after three years of good work for not being sexy enough for Mr. Berman's standards may have triggered what happened later. I think that this seem to be a plausible explanation.