Those biomes are still very much under construction. I'm pretty sure you were running the code which was basically slapped in for preliminary terrains in the biome mods. I'm actually working on a new terrain type to generate Plateau, and I think the default code I just took out was the canyon code, which does things like that.
That's more or less the timeframe, yes. The Highlands terrains were the first I did and they're certainly usable, but I pulled them back from draft release status as I've developed a number of techniques like jitter and smooth hill transitions that I'd like to try on Highlands, as it is my own favorite biome mod and I want it to shine. If you're a Highlands fan, you could help it along by looking at the biomes and making suggestions on how to improve them. I'm particularly interested in whether there are too many ugly deadvallies and whether they compare poorly to some of the BoP biomes, especially DeadSwamp, Chaparral, and Mountain, which all use the jittering technique to produce effects I couldn't do before.
Right now my plan was to finish the EB and EBXL biomes and go back to redo Highlands later if I have time. But I might be persuaded to finish up Highlands first with some help.
Yeah Highlands really has some of the best biomes available right now. But an updated version for 1.7.10 would be really nice and not a so-so working version for 1.7.2. Especially the integration part needs an update as many of the supported mods have been updated and some even several times.