Hollywood report has a great account of what all happened:
https://www.hollywoodreporter.com/movies/movie-features/rust-armorer-fair-shot-trial-1235823841/
She had hired to play two roles for them to save money. She was told on the 17th she was spending too much time being an armorer and that she should spend more time with dealing with props. On the 21st when shooting started, she was clearly in the role of Armorer:
"The prosecution will say there’s another reason Gutierrez-Reed wasn’t able to do her job properly: drugs. Prosecutors intend to introduce texts from Gutierrez-Reed’s phone that indicate she was using alcohol, marijuana and cocaine during production. On Oct. 17, the night she asked Pickle to approve more armorer days, Gutierrez-Reed was communicating with a driver on the film who asked if she was still up and if he could stay in her hotel room in Santa Fe before getting back to work in the morning. “I’m up, I’ll roll one,” Gutierrez-Reed replied. “I always gotta stay up for a bit when I get back anyways.”
In other words, she had a dual role and was informally asked to spend more time on the non-armorer task. This is way different than saying her job as Armorer had finished. No way they could keep shooting the movie if this was in fact the case with no other person playing that role.