Hmmm... Yeah, I actually dread the entire explanation that she will give because it will be ridiculous, far fetched, and unbelievable (no offense to Drifty) because every scenario we've thought of has giant holes in it. The only way that she could explain anything with it making an ounce of sense as to why she didn't tell the MC is if they kidnapped her family or his family or something along those lines. Not explaining something and leaving "for his own safety" (that just grinds my gears) is not a forgivable act on its own. There has to be something more to it, someone else she loves or is connected to being potentially harmed. Because right now the "I love you desperately but have to leave for an indefinite amount of time to solve a problem kinda related to you" is bullshit.
I'm really curious about this explanation too, but as u said, i fear it will be unbelievable or full of plot holes, thought kudos to Drifty if gives us some believable, good story.
Yeah the kidnapping of her/his family or something along these lines would be a good reason, the only problem i see here is:
The problem didn't come up suddenly, Steph seem conflicted about it for quite some time and in the note she speaks from making ''them'' accept the Mc and that ''they'' would hurt the Mc... the only one which seems in any kind of danger is the Mc for whatever reason.
We have the rich father from the middle east, but he seems to have no interest in Mc and the mother was for all we know only a one night stand...i mean he gave the Mc some money...but if he's one of the really rich in the middle east, 200k is more like a small tip for a good conscience than anything more.
Also probably no one, or only a handfull of people would know of the conection beetwen the rich father and the Mc.
And would it even make sense to target the MC to harm/blackmail the father, which doesn't seem to care about the MC.
So most resonable explanation i can think of, it has something to do with Stephs family, probaly some kind of crime family/cartel, at least that could explain why only the mc is in danger and not Steph and why she would say, she makes them accept him/their relationship...
But man, whatever the reason is, this will take some really good writing to be convincing.