Length code not recognized by Plex

I've just started using Plex, a local and remote (over the internet) jukebox program. One hangup I've encountered deals with the length of a track.

Plex correctly displays the length of most tracks, but with live concerts, I use MergeMP3 to create a gapless version as well. Unfortunately, that program carries over all the tags from the first track in the sequence, so Plex displays the same "Duration" for the complete concert as for the first track (once you start playback, it does show the correct length on its internal player).

I fired up MP3Tag to change the value in the duration tag, and found that it didn't have a duration tag but did had a Length tag - which already had the correct value! I tried renaming it to Duration, but that appeared to only apply to MP3Tag's onscreen interface.

Help!

PS Plex is a free server/client program that runs on OSX and Windows, with free client apps for Roku and simillar devices and $5 clients for Android, iOs and Windows Phone. The home site is www.plexapp.com. It automatically imports artist bios and photos and album art from Last.fm.

Since there's been no response to this question, despite a number of reads, I guess I'll just have to start putting the real track durations into the titles of the gapless tracks so I don't have to start playing them to find out how long they are. Inelegant, considering the length of these track titles, but it'll have to do.

Did you ask the Plex support?

For mp3s there's the official Length tag that stores the length in milliseconds.

Does Mp3tag show the correct length?
If not you probably need to fix the vbr header of these files.