Hello there!
I've reached out to the development team regarding this issue, and I have an update to share with you all. The development team has confirmed that this is working as intended. If you are in a relationship with another NPC (such as Randvi), you will be given the two dialogue options: "I'm with someone" and "I think of you as a friend." After selecting your choice, you will be able to interact with Petra again. If you are single, or have chosen to break up with your current romance, you will have a new dialogue option available to start a relationship with Petra instead.
It seems that this "infinite" conversation is allowing you to start a relationship with Petra at any time, if you aren't already in a relationship, rather than locking you out of the opportunity after finishing the initial conversation.
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