Does your fertilization program include minor elements?

Yes, atleast the packaging says so. The current one is 13-40-13 plus micro elements such as boron, iron, manganese, molybdenum, zinc, copper. They were transplanted recently and I give them more phosphorus because of weak roots and thin stems.

A friend of mine thinks it's either magnesium deficiency or leaf mold due to the high humidity inside the greenhouse however I regularly open the windows for air flow and seal it for the cold nights only.

How large are the plants, and are they going outside eventually?. The dark purple veins are also noticeable.

Most are about 12 inches tall. Not yet. I suspect the cold for the purple veins. The night temps are around 40F outside however I did some upgrades to my greenhouse with bubble wrap and small electric heater with thermostat. The average temp inside is now 60F at night and up to 95F during the day.

How long have they been on that high phosphorus fertiliser? Do they all have the problem or just one?

Do the blemishes go through to the undersides or anything underneath of note? It’s hard to tell at this early stage but I would look at cultural rather than disease at this stage.

It turned out to be an early sign of tomato leaf mold(Passalora Fulva). It started spreading to other plants too. Some of them started going brown.

@SW I started giving them 13-40-13 last week. Yes the undersides are also affected.

I haven't really considered this but you may have point. I will give them a break for few weeks.