I have yet to get a single verdant sprig from a mod-added crop from a number of other mods(Roots 2 and Pam's Harvestcraft are the ones I've tried); any sprigs I've gotten have all come from wheat, carrots, potatoes, or beetroots - crops from the base game.

They also do not seem to drop when using mods which allows for harvesting these crops via right-click, at least not with the mod I'm using for it(Harvestables on Curseforge right now, or Harvestcraft when I had it installed in a different setup). They still occasionally drop when the crop is broken when fully grown, but it'd go a long way for the mod's usability if it worked with right-click harvesting mods.

The right click issue also applies to infernal stems from nether wart, if a mod supports right-click harvesting for nether wart, I still only get infernal stems by breaking the fully grown crop.

Yes that is correct. Currently they only drop when the block is physically broken.

I'll see if theres an easy way to get it hooked into crop harvesting too.