Seems I haven't been watering the plant enough for a while. The pot gets really light less than 24 hours after each watering. Previously I watered it every other day, now I'm giving it 2.5 liters daily.
The initial plan was to do a 6 week veg, but the plant is still a bit short, even for my tent. I'm hoping next week will be the last before flipping to 12/12.