I also would love to know.
One month ago I come up with similar situation, my chaeto become to tight and curly like a brick, even with nitrates and phosphates. Early was long and fluffy.
What changed in my case: I use a diy 2 parts with traces from Fauna Marin. However I was dosing about 5x times the traces in the Calcium solution.
Fauna Marin balling is much less concentrated then Randy's 2 parts.
I reduced the traces to be comply with it.
Result I got a similar state as you.
I read about dosing and macro algae and come up to some things like:
https://www.marineplantbook.com/marinebookpage10.htm
The chaeto reator Pax Bellum Arid also have this bottles of traces which include iron and manganese.
And what is Triton putting in the first bottle, I think the magnesium one, that does not put in the case of supplements /other system. I would love to know.
From two weeks I made a new Calcium bottle with more traces, bottle 2 of Fauna Marin which has iron, manganese, zinc, copper. And also daily dosing of iron. The chaeto is now growing again with long strings.
But I maybe risking putting other traces too much... I don't know.
I have one question, while you were away (I hope you are fine now, we missed you) , did you stop feeding the fish with nori? I suspect the feeding consistent parts of nori we are giving the traces chaeto and general macro algaes need. I don't because I have a nano so my fish don't require that.
Finally it would be interesting a ICP test on your tap water. Then we could see if any of the traces in the site I mention are there.