Usually you hear about actors leaving shows. In this case, it's the opposite: "Matlock" left Julie Sommars. Technically it left Los Angeles, and Sommars wasn't willing to join it.
Her final episode as a series regular came at the end of Season 6. It also came at the end of "Matlock's" run on NBC. After that season, the show moved to CBS. In making the deal with the new network, the producers also made a deal to move production to North Carolina, series star Andy Griffith's home state (which is at least on the same side of the country as the show's Georgia setting).
As a result of the move, the show also lost Nancy Stafford, who played Michelle, Matlock's law partner and friend.
"Both of them have families in California, so we just accepted their wishes," Griffith said at the time in an interview with the Fort Lauderdale Sun-Sentinel.
It's possible that Sommars may have wanted to spend more time with her family -- her husband, two sons and daughter -- anyway, and just took the move as an opportunity. Afterward, her only roles were (as you say) one return appearance in the show's ninth season, as well as a guest spot in the similar mystery series "Diagnosis Murder," both in 1994.
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