Yellowing leaves on 2 plants with yellow leaves at bottom dropping and dying?

Was the pot really light when you got back? Did you have a means of watering while you were gone? I had something like this happen to me once when I let the pot get too dry. Turned almost all leaves yellow in 2 days. Without enough water plant dries out and eats itself for nutes. If water was present, I agree with others in checking ph and ppm. Either could also cause nute issues. Hope this helps, best of luck. Happy Growing!

Could be magnesium. If you have epsom salts around the house, try a teaspoon in a liter of water and toss that in. If it's magnesium you should see it start to green back up within a few days. But you need to be damn sure your soil pH is 6.3 to 6.6 in between those numbers. Any grower that has no way to check their soil or water pH, just shouldn't be growing, period.

When u feed the plant does it begin to rise its leaves? Then underwatering Otherwise heat or lightning stress, you can use a phone app to measure lux or use the back of ur hand as an indicator, if you feel more than slight warmness its too much for the plant Your stems are very purple too, did u check runoff-water pH? Could be a nutrient-imbalance also