Answer: It's really difficult to know without seeing the tree, but based upon your description, it sounds as though the spot was probably a wound of some kind - perhaps a borer was investigating the trunk. In response to the injury, your plumeria oozed some latex to protect itself. The split bark indicates a lot of sap (or latex) activity - perhaps from additional wounds, or a reaction of the tree to the original wound. At this point, especially if you're getting new growth and the rest of the tree looks okay, I'd just leave the wounded area alone. A healthy tree will compartmentalize the damaged tissues and eventually callous over the wound.
Best wishes with your landscape!
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