It is a 1 in 8 chance to get the berry (there are no seeds) so there is a chance you just got unlucky so far :( If it's a bug, I haven't been able to recreate it. I've made the odds a little better now and made it so that you automatically get at least 1 berry while foraging if you haven't gotten one past day 15. (Which sounds like you are already past) Let me know if this helped!