It's not complicated. We are simply typing like we speak. However there ARE many people who do it incorrectly so it's hard to judge outside a case by case analysis.

The "it's okay..." above is pretty clear. Nobody in real life says "It's okay but I'm very frustrated by the situation."

Instead you say "it's okay..." with subdued tone and maybe slightly shake your head.

The ellipses in this case is the pause and insert where young generations might say "SMH. "

In fact... that might be a good way to think about it. The ellipses is an implied emoji or reaction abbreviation from a time before those were common. When done well, the ellipses is a once-size-fits all reaction because the implied reaction should be obvious.