Artful Codger: what kind of music do you play that has so many Lydian tunes that any feature of them can be described as "common"?

authentic and plagal modes don't generally apply when discussing modal folk music, because folk tunes aren't artificially limited in range

With older tunes, you often have relics of tetrachordal or pentachordal structure. A song tune that started life as a tetrachordal melody with the tonal centre at the top will usually develop by adding an upper pentachord - that gives you a plagal result. The process is best documented in Indian music but it occurs much more widely.

Artificiality is more likely to be involved in extending the range than limiting it, as when somebody with a new wide-range instrument or a wider-ranging voice than the norm gets hold of a tune and adds some flashy stuff of their own.