Playing on PC.
So I went to go do some bounties and stuff for Spider and noticed that Petra wasn't in the corner. I thought this was a glitch so I went to go do Spider's bounties and other stuff for maybe 2+ hours. I go back, and she's still gone. I did some experimenting and I put a tracking thing on her and walked out of the area to see what would happen. The marker pointed to inside of Spider's Safehouse. Right as I enter the area, it shows she's there for around half a second but then vanishes. No clue what caused this to happen.
From [url=https://www.d2checklist.com/4/pizzatime%2311143/triumphs;id=516895854]here[/url], I can see you have completed the Forsaken campaign.
[spoiler]Petra will move on to another location as you progress with the story...[/spoiler]