I had some other problem. I encoded an almost 3 hour wav with as VBR with a minimum of 128kbit but the first frame is a 32kbit frame. This only occurs with this file (version 3.87).

I think that's a feature. From the manual:

When used with variable bitrate encoding (VBR), -b specifies the minimum bitrate to be used. However, in order to reduce space wasting, during silences the smallest frame size available will be used.

Borislav