Two years ago Roblox made the sensibly most backlashed (on the DevForum) move of all time to “temporarily” freeze marketplace support for audio when they implemented the new privacy system. Unfortunately the 6-second cutoff only applied to audio uploaded prior to the change.
It is indeed a Roblox issue where private audio will show a zero’d out length but may still play the full length of the asset if you whitelist it to your game by inserting it from the inventory section of the toolbox (copying and pasting the sound id will not work).