Yes sunburn is also my best guess but may I give my 5 cents to it?
I would taking care when watering that you not water the stem.
Make a heap around the stem and then a ditch or just stick two cut plastic bottles beside in the ground about 6-8 inches away from the stem.
Tomatoes hate getting wet knees..
In modern times is going the right way forward the only way to coming back to nature.
posted 5 months ago
I've never heard that we had tomatoes for years and always straight watered them. Thats interesting...ty