Acquiring 'barbarian settler' through heathen conversion?

[mfly6148]

Of course, barbarians don't really have settlers, do they? But they capture settlers from civs quite frequently. (In fact, in my current game I'm quite sure they've captured the *same* settler, from Polynesia, multiple times.) When defeating a barbarian unit that has custody of another civ's settler, we're given an option: either return the unit to the civ ('for which they will be very grateful,' etc.), or else keep it - but as a worker. With the heathen conversion reformation belief, missionaries convert barbarians. But what happens to the newly-freed 'barbarian settler'? I mean, what happens when my military unit - let's say it's an archer - defeats the barbarian military unit that was guarding the captured settler? The captured settler shows as red (like all barbarian units) and the mouse-over text even says, 'barbarian settler.' But if I have a missionary in the vicinity, who is equipped with the heathen conversion belief.... *sometimes* the settler is converted, and I've now acquired - free of charge, with no prickly question about whether I want to return the unit in exchange for some civ's gratitude - a new settler to expand my empire! This can be quite nice, when it happens in some remote corner of the world where I'd be unlikely to send my *own* settler (even moreso, if it happens near a natural wonder that I'd like to acquire, as in my current game). But not always; and that's what's confusing me. I'm in this situation right now, with a liberated 'barbarian settler' and my missionary nearby... but no conversion is taking place. And that's happened before, in similar situations. Anyone understand the mechanics for when a 'barbarian settler' can be converted, and when not?

[mfly6148]

EDIT: another variation on the scenario, above, is when the barbarian military unit that is guarding the captured settler is itself converted by my missionary; it has now joined my civ, and has custody of the settler, which still shows as 'red' (barbarian). Next turn, I move the newly-converted military unit a space or two away from the barbarian settler, bring in my missionary, and... bupkus. No conversion. I'm wondering if that's the difference: whether the barbarian military unit guarding the settler is defeated in battle by my military unit, and *then* my missionary swoops in to convert the settler, or whether the barbarian military unit is itself converted by my missionary...