I had two of these for around a year as well and they always struggled no matter what I did. I didn't get any brown spots on them but they were definitely prone to chlorosis and the tips and edges of leaves would often turn a bit brown and/or curl upwards. Additionally, they always seemed to drop their lower leaves and sometimes would drop all their leaves before a heavy push of new growth. They seemed very prone to overwatering and always seemed to want more iron. I had them in amended promix hp and used RO water for reference.
Here's one of my seedlings from earlier in the year. Growth appeared healthy and it was pushing to the point where it was falling over, but within a month all the leaves had fallen off despite looking healthy, and they never came back.
If I tried again I'd probably experiment with a gritty mix and keep them in smaller pots for longer as their lateral roots seem to develop quite slowly, sort of like a garcinia. If you can get in touch with Anderson Tropicals I know he has/had some of the largest and best-looking seedlings for this species, so maybe he could provide advice.