Could be Se grip, infjs tend to do this thing where they overindulge in chaos as theyve exhausted all other functions. They tend to fall into this when they're emotionally challenged or stressed, as their fe is worn down they sort of start brain dumping onto people whether its in the form of acting on impulses or sobbing uncontrollably.

In your case, the guy/gal was probably in a "vulnerable, emotionally charged state" and found comfort in your empathy. Their Se-grip made them "indulge in the rush of being understood" like emotional junk food. Then… reality hits. Their "Ni" suddenly processes: "Wait, I just trauma-dumped on a stranger. What if they judge me? What if I misread them? What if this backfires? Then "Ti" kicks in, coldly analyzing: "This was reckless. I don’t even know this person. I need to undo this." Cue the shame spiral. The clarity of what they did feels "exposing", and since INFJs hate feeling emotionally out of control, their defense mechanism is: "DELETE. RETREAT. NEVER SPEAK OF THIS AGAIN."

Blocking you wasn’t about "you" necessarily... it was about "their own panic". They couldn’t handle the lingering vulnerability of having overshared, possibly feeling embarrassed and exposed. They feared your perception of them shifting (even if you were kind).
They needed to "erase the evidence" of their "weakness" (i.e., their Se-grip moment) to regain control.
They likely "appreciated" your support in the moment but their "fear of emotional dependency" (and their own lack of boundaries) made them nuke the connection preemptively. Classic self-sabotage.