Do we know there wasn't more to it? Actors leave shows all the time for a variety of reasons.
That Lien barely worked after her exit from Voyager suggests there may have been factors other than the quality of her work on camera which led the series producers to part with her after the third season.
I wonder if those linking her current substance abuse issues with her exit from the series have things the wrong way round.
Before Voyager, Jennifer had a lot of work in TV-series and voiceover.
As for the infamous "drug rumor", back in the old days when I was very p***ed over the fact that Kes was written out and Jennifer kicked out, I did a lot of research on this subject.
I didn't find any evidence at all that Jennifer had been kicked out due to drug problems. In fact, the "drug rumor" had started earlier about another actor but for some reason it spilled over to Jennifer after she was fired.
There have been other cases when people have been fired from shows for different reasons and in those cases it has become common news why they were fired.
When Shannen Doherty "left" Beverly Hills, it was common knowledge that she had been a pain in ther a** for almost everyone involved with the show.
When Gary Dourdan "left" CSI, it was also common knowledge that he was kicked out due to drug abuse.
Same for Charlie Sheen when he was dumped from 2 1/2 Men.
But I've never found any evidence that Jennifer was dumped because of doing drugs then.
Her long absence after Voyager, her weight gain followed by a fast weight loss and over again might point at a depression caused by being kicked out from the show.
I guess that she might have had some mental problems before that but being kicked from Voyager might have been the first heavy nail in the coffin.