The multi-extension stuff only applies to certain patterns. So .part1.txt and .part1.rar are treated differently, for example, since it's how multi-part RAR archives are named and you want the "extract to <archive name>" command to trim off the .part1 when creating the new folder.
There is indeed a difference between how {file|noext} and {filepath|noext} handle multi-part extensions. That probably wasn't intentional, but if we change it now it might break people's buttons, so I am not sure if we will. But explicit noext_m and noext_nom or something might make sense.
You can use {filepath|..}{file|noext} as a workaround, to get the full path minus the multi-part extension.