I am trying to get a random series of lengths from the list (4/14 5/14 6/14) to span over a certain time, in this case 4 bars.

(length-span '4 (rnd-sample 40 '(4/14 5/14 6/14)))

The result keeps including tied sixteenth notes, although I want to limit the score and rhythm to two eighth septuplets per bar (with ties).

I also tried to include quantize as a second grid which didn't help to "clean" the result.

Am I making a mistake here or is this some kind of bug?

My results are different. What version you are using?

 

(length-span 4 (rnd-sample 40 '(4/14 5/14 6/14)) :omn t)
=> (7d. 7d = 7d. = 7d 7dh = = 7d. 7dh)

 

With random selection it is possibile, at the end, you endup with 1/14 value:
(2/7 3/7 2/7 2/7 3/7 5/14 5/14 2/7 2/7 2/7 2/7 5/14 1/14)

Thank you for the hint; I now updated my version and it works just fine.