Thanks, @Modelman, I feel less alone 
This may get fixed for the next version.
I try to change the transition with the config preset:
{"on":true,"bri":255,"transition":30,"mainseg":0,"seg":[{"id":0,"start":0,"stop":144,"grp":1,"spc":0,"on":true,"bri":255,"col":[[255,160,0],[0,0,0],[0,0,0]],"fx":72,"sx":5,"ix":72,"pal":50,"sel":true,"rev":false,"mi":false},{"stop":0},{"stop":0},{"stop":0},{"stop":0},{"stop":0},{"stop":0},{"stop":0},{"stop":0},{"stop":0},{"stop":0},{"stop":0}]}
The transition number is correct in the WLED interface, be presets still jumps from presets without any visible transition.