Yeah, I don't know about that. I did notice that when I went into the drug farm for a collectible well after I completed the story, it hadn't respawned for me either. So either it doesn't actually respawn that often or like you said it stops after the story ends. I'll have to check on that on my next playthrough.
Not that it really matters though. The first and only time I raided it they only had like 15K, which wasn't even worth my time. Even at 100K it wouldn't make any difference, particularly in the postgame.
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