sfizz loop_type alternate not working?

trying to make sfizz loop my samples forward <-> backward

by adding:

<global> loop_mode=loop_continuous loop_type=alternate

but the samples keep playing forward only, regardless of loop_type=backward, forward or alternate

also tried this:

<global> loop_mode=loop_continuous loop_type=backward loop_start=0 loop_end=50000

with the same result.

how should i go about this?

thanks!

robin