So I’ve been helping my friends beat the forsaken campaign and many many times we go to the shore and Petra is not next to spider like it indicates on the radar.
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I´m just gonna take a wild guess and say you´ve unlocked the Dreaming city already. Now considering that petra leaves and you meet her @ the Dreaming City, it is safe to assume that you being in this fireteam causes her to sometimes bug out. Nothing really to cry over