Normally police stop saying things about cases because they run cold. Ann may have also got lost in the thousands of MP reports filed. I believe the same thing happened to Sherri Jarvis (Walker County Jane Doe).
An alternate theory is that police found her or at least someone claiming to be her (probably her as her appearance is quite recognizable) and they left her in Plainview. I believe Ann may have continued her journey after Plainview realising her family may try and contact her. She leaves Plainview making her whereabouts unknown, making her just as missing as after the Plainview sighting.
Also, the rehashed articles are altered to fit the time. 1959, instead of saying "missing for 1/2 years" it says "missing for 3 years". These articles also speak nothing of the Plainview sighting. If Ann was found and in contact then why would they rehash false information? There was also new information in 1959 like the hotel sighting:
Caldwell hotell 1959 - Newspapers.com
This sighting was in October 1958, after the 1957 Plainview sighting. Ann was possibly seen using a fake name in a Tucson hotel. No word of her being found before this. It says "missing for 3 years" just like the rest of them. If Ann was found it would have to be in the early 60s, not 1957.
You can stick with your found in 1957 theory but I don't believe it. There's too much new info after the sighting for it to be the end of her case.