I'm not 100% certain, but I believe the 3 Slot Forge mod is making it tricky to actually take things out of the output of the Forge. It seems like the cursor decides on a whim if it's going to highlight it to take or not. It's not all the time, but rather about 80% of the time. The interesting part is, sometimes it seems to flash like it wants to highlight it (When mousing over it) and then decides either yes, I can take it, or no I cannot.
Love your mods in general and am excited for DF A19. Just wanted to post this to make you aware - again, can't say for certain it's the 3 slot forge modlet or not - But I've only got the vanilla game, +3SlotForge, BackpackButtons, HP Bars, and Max Animals/Zombies installed.