I'd tend to think over watering or poorly draining soil. It looks like you have a lot of slow release fertiliser in the pots so there shouldn't be any deficiencies.

It may have been indeed magnesium deficiency because I'm now finding them starting to grow again since these pictures were taken a couple days ago, and it doesn't look like there has been any further damage since it happened.

It may also have been linked to over-watering especially since some of them were transplanted into their own pots, and I wanted to ensure they had enough water at the time before the roots were settled in.

FWIW...I had a very similar problem with my titanums until I was able to place them in my outside bush house with natural sunlight and (probably most important) natural air circulation. Even when our night winter temps threaten to drop below 10C and I move them indoors for protection against possible frost, I still place them outdoors during the warmer daylight hours.

I'm wondering if the leaf spotting could be a fungal problem(?) Good air circulation is a safer solution then fungicides. Maybe try a fan along with those lights if your climate is too cold for the "great outdoors."

The newest leaf on my titanum is approximately l meter tall. The plant keeps producing larger and larger leaves without even looking like it will go dormant!

As it is I'm getting plenty of potential cutting material, but I really don't need another of these giants. I'm just hoping that the plant will go dormant come next year and the beginning of the colder winter weather. Anybody else experience the constant leaf production from one tuber?

I have also found that many of these guys have continually produced new and larger petioles. They sometimes make small ones too, but overall they get bigger and bigger, and I haven't seen them exceed 3 at any given so far.

Right now, I only have a couple titanums left, not even the largest ones. I have sold off 30 something plants to a couple dozen people, perhaps 40% in southern Florida, and mostly through the south through the summer. The largest reported size I've heard of so far is around 2 1/2ft petioles and 1 inch stems which is pretty decent for 10 month old plants, but most likely only possible because they were strong plants to start with that were sent down south where they get more heat and humidity.

The ones I still have are no more than a foot tall and this is to conserve window space as I can't have tons of plants to over-winter.

This is the latest one that I sent out around 2ft tall with 3 petioles, and a 4th one forming, likely getting to a decent size based on how it looks now. The corm appears surprisingly small, less than the size of a tennis ball, but larger than a golf ball, but likely because the 3 new petioles were just produced a few weeks ago.