I've always found it interesting that Austin was such a big part of KNOTS for several seasons as Jill, but yet wasn't in the opening credits until Season 10. And that was right before her character was killed off. Show business politics I guess at work there.
I have no way of knowing about her situation for certain, but there are many cases where actors prefer to be recurring, non-contract players rather than be tied down to one role by a contract. Yes, the contract guarantees them a paycheck and a place to work every day, but it also limits the actor's options to accept other roles, to work in other venues, and to travel or do things important to them. Being on recurring status allows an actor to come and go as they please, work as much or as little as they like. She likely had opportunities to do work for Canadian TV that would not be open to her if she was under contract with KL. Her character was prominent enough that she got to work frequently during her recurring years, so it could be said she had the best of both worlds.
In order to do the story of Jill's descent into insanity and death, they had to sign her to a contract because they needed to cover their bases. As a recurring player she could in theory walk in one day and say "Sorry, guys, I got offered a big lead role on a Canadian series and I quit." Signing her to a contract "nailed down" her availability. They could then write the story with confidence that they had Austin for the duration. Since her face and name continued to be in the opening titles even after Jill bought the farm, I assume (but don't know for sure) that TA was given a one-season contract and got paid for the whole season despite leaving when Jill died. If she had only signed on for X number of episodes, her image would have been removed as soon as those episodes had aired (think: Constance McCashin and Julie Harris being removed after four episodes in S9). Offering to pay her for a full season without having to work a full season might have been a nice enough incentive to get her to stay on and do their story of Jill's exit.