After I ran qsynth the graph looks like this:

I had to zoom out to find the new node. Now it has to be moved manually.
qjackctl places nodes very far from existing ones.
It should choose a more reasonable position.
Eben if the item of the same kind has been previously added - the previous position should be ignored when it is far from currently displayed nodes. (I am not sure if that's what the problem is.)
I remember reporting this a while ago, but somehow the problem is still there.
qjackctl-0.9.10
FreeBSD 13.1