Off the top of my head, I can recall seeing this treatment in the Book of Kells, so it isn't an Art-Deco-only phenomenon. As far as a good name for it, the title of your post seems to cover it...

…couldn't get the picture to upload..

Remove the space in the file name.

@Nick Shinn: thanks!

@Nick Curtis: Ah, that would explain why I always thought this style looked kind of runic. I think this picture probably shows the one you refer to. http://jonsullivan.com/images/ireland/it-kells-bg.jpg So maybe it's possible that this treatment was brought into popularity in the late 1800 – early 1900's through the arts&crafts movement, inspired from a celtic tradition?

If I'm not mistaken, the Book of Kells image shows a «diagonal» that's both moved down from the top and moved up from the bottom, so completely avoids pointy joins – sort of like a H with a tilted crossbar. The one from Korinna shown above can then be seen as a hybrid between that shape and the more usual one where the diagonal is, in fact, a diagonal, forming an apex and a vertex.
So much for observation. I'd like to learn more about the background of these forms, too.

It's more likely that the elongated aesthetic of art nouveau was interpreted in lettering as "high-waisted", and that this form of N harmonized with the look of capitals that have raised middle crossbars.

Also, in ultra-condensed low-contrast styles, squeezing in a long diagonal crossbar is extremely problematic, so the higher join lessens the angle, and the problem.

When designing Beaufort, I found that this effect enabled me to keep the angle of the diagonal consistent across weights. Also, it provided a bit of interest to that bottom right joint in the heavier weights, to match the interest provided by the sharp point, descending below the baseline, in the lighter weights. A normal blunt join would have been too dull, and a less-than-stem-width flat terminal would have been an anomaly. And, also, it counterbalances the "overshoot" of the diagonal at the top left.

IMO, individual glyph shapes in original type designs are most often functional interpretations of the thematic vocabulary, concerning harmonies of shape and proportion, that characterize typefaces, and not primarily references to specific glyph forms associated with historical precedent—although previous themes in toto may inform the new typeface.

sorry for the late answer, I was away a couple of days...

Nick, it seems like you summed it up pretty well. The the raised diagonal certainly works better in condensed design, as they fx were quite popular in the late 18 – early 1900's and this is definitely a more plausible reason for its appearance than a purely stylistic inspiration.

Thanks for your help!