That is peculiar alright.
I only have two reccomendations.
1. Takl to Multitech about it, to see if there is a configuration error.
2. Check out this link: http://www.d2tech.com/vp_alogrit.html
Which is for the “VP Open Algorithms Libray”, and discusses both Codecs more in detail.
Are you sure you have header compression set up the same for both, and that you are only having one conversation occur when G.723.1 Codec is on?