Fractal personal level not increasing
I have the same reaction as do respondents in your reddit thread: the more likely scenario is that your party exited from the specific fractal before you were awarded credit — you can loot the ground chest before getting completion credit (that’s the part that awards karma or +1 personal level).
The image you’ve linked merely shows you were there to defeat the boss; it doesn’t show that you were there for official completion.
I’ve done lots of different fractals with people at or below the island level and to date, everyone always gets credit (since at least one of us in the party refuses to press the ‘ready to leave’ button until we’re sure everyone has looted and we’ve gotten our karma). I have friends who routinely leave early from the fractals with delayed completion credit (Maw/Ocean is the worst for this imo, but Mai Trin & Cliffside — the two to which you refer — are pretty bad, too).
Of course it’s possible that a recent patch introduced a new bug or that you are one of the few (unlucky) ones encountering a weird “edge case” that has always been present. However, it’s more likely that you (like many others before and after) are getting caught by the unintuitive delay between getting loot and getting completion credit.
tl;dr next time you’re in a fractal, wait (and if you’re concerned, take screenshots showing time stamps before killing boss, after looting ground chest, and after waiting).