Kelp grows, gets clipped by the pistons when 3 high, and flows into a pair of smokers all at a steady clip (it's a pair of smokers because my farm is slightly wider than the model, and the kelp WAS backing up with a single smoker when I built it under 1.15). Kelp does not now, nor did it ever, grow fast enough to keep the hopper full before feeding it to a pair of smokers. I will try delaying the feed, changing to a single smoker, and swapping the smokers out for furnaces to let it build up (I could also dump a huge amount of kelp in there to basically prime it). Expanding would be a little bit of a pain, but who doesn't like yet another build?
It's a multiplayer server and the world file is many gigabytes, but I'll see what I can do.
Reminder: it worked fine in 1.15, and I haven't made any design changes since then.
Thanks much for the help.