Confbridge and Speex UWB, bad quality, only 42 bytes RTP payload

Hi there, I am using Asterisk 13.1 and I am stuck using Confbridge together with speex 32kHz codec. Channel info shows the following:

NativeFormats: (speex32)
WriteFormat: slin32
ReadFormat: slin32
WriteTranscode: Yes (slin@32000)->(speex@32000)
ReadTranscode: Yes (speex@32000)->(slin@32000)

So it seems that my sampling rate is 32khz througout the conference, so no downsampling is done here. If I trace at the side of the callee, I can see that speex UWB is chosen as codec, but the confbridge seems to use one of the lowest quality settings possible, since RTP pakets coming from confbridge are only 84 bytes big, leaving only 42 bytes of payload.

If I have a normal connection without the confbridge, I get good quality and more payload per RTP packet.
Is there something I am missing in configuration or is this a known bug?

Thanks for reading!