They were shocked and said something along the lines “they would never have ever expected her to steal from the company” So THEY viewed it as theft
They should have not have said that. At that point, they hadnt investigated
But you clearly, gave judgment when you told them. Thats not your job. Your job is to flag anomalies. Not to pass judgment on what crime or employment terms and conditions had been breached. If you hadnt they woild have responded with that.
Also that's not confirmation that she stole anything. That's them speaking out of turn, before and investigation has happened. So no one could confirm anything.
It's quite clear you present this as a theft. Wether Anna did, or didnt, they believe hee story. The problem is that because you decided to make a judgment on Anna and her motives, its caused tension on the team.
Let's look at this the other way. You have worked somewhere years, often trusted to do it and buy items for lots of different reasons. Someone starts work there and tries to put a case together to accuse her of theft, because you have done what you have always done. They take it to the management and presents it as theft, trying to heavily influence management. Management believe you, so that's fine
But would you want to work with this woman?
From what you have posted, its clear you think it was theft. That's what's causing the tension. The problem with that is wether you believe it or not, management have decided it wasnt theft. They believe her. So either swallow your pride and make an effort with her, or accept your time in this job will ne difficult.
Like it or not. If you want ro stay in that job, you need to Try and clear the air with Anna. And dont make judgments on why anomalies have occured. Just report that you cant get the payment to match up.