Seeds are capped at 200, and were also in 40d.
If I still had it, I would upload the screenshot where I had in excess of FIVE THOUSAND plump helmet seeds.
Legendary famer/brewer ftw.
Okay, let me restate that. The production of seeds is capped at 200. You can buy as many as you like, which says immediately that the number is not kept in a one-byte variable. If you did in fact make 5000-plus seeds by the combined actions of a legendary farmer and a legendary brewer, then the capping mechanism was not at that time working correctly when more than 200 seeds were produced at one time. I have never seen it happen in one of my fortresses, which usually have legendary farmers and brewers after a few years.
If in fact your 5000-plus seeds were produced by an oversight, then correcting that oversight might result in zero seeds being produced whenever the action was calculated to produce more than 200 seeds. This would also be an incorrect method, and I doubt that is happening, but I'm not in a position to know how Tarn has coded the cap on the production of seeds. In any event, it should be happening for all seeds produced by brewing, not just for one type.
A possible cause might be that
weaving plant processing is not producing seeds. Amongst all the Dwarven crops, pig tails are the one that can also be used for weaving. A loss of seeds through weaving would be a drain through which all the pig tail seeds could eventually be lost. I haven't noticed losing pig tails as a crop due to weaving, but the production of fabric has had much lower priority for me in the newer versions because of the lack of a use for clothing.
Re production cap on seeds:
http://df.magmawiki.com/index.php/23a:Seedhttp://df.magmawiki.com/index.php/40d:SeedEdit: corrected which workshop makes thread from pig tails