I started my fortress using the Crimson Crops mod (increases growth time, reduces yield, season dependence moved topside, domesticated animal production rate reduced), but gave it up after a couple of decades as half of the surface crops didn't work. 10 years or so down the line I suddenly ran out of pig tail and cave wheat seeds. OK, I harvested some wild ones and got me a seed supply, but one year later, I am again out of seeds (with cave wheat, I have a single tile that has cave wheat for a single season each year, so they shouldn't even have been able to plant the seeds I collected). This discovery was done late autumn, beyond the planting season for both pig tails and cave wheat. I again collected pig tails and cave wheat, brew and threshed them respectively, and verified I did have seeds (9 pig tail seeds, 10 or so cave wheat). Seed cooking is off (verified), and I also haven't done any cooking since. Come spring I check my seed stocks, and I don't have any seeds of either of those crops! I've increased the total seed cap from 3000 to 10000 to ensure I can have the maximum number of seeds of each crop I plant (each is planted on a single tile for a single season, with the exception of plant fiber producing ones), and the number of seeds is 3400+, so that should be fine, and the cap culls production of new seeds, as far as I know, not destruction of existing ones.
Any idea of whats going on?
I've had similar problems earlier (0.40.24) without any mod, where I've brought garden vegetables on embark to get seeds, and end up not having the vegetables, nor some of the kinds of seeds before I start using the kitchen for the first time. On the next embark I didn't have the problem, though... I believe others have had similar problems.