Hi,
im a biochemist and here is my theory, propylene glycol is metabolised to lactic acide in the body, this results in a decreased PH (more acidic) blood or
lactic acidosis, the shift in ph has an effect on minerals in the body and the metabolism of minerals since minerals can only bind at a specific PH. This would result in a loss of Mg++ ions which in turn causes cramping and the twitching you are describing. My advice would be to take a Ca++ / Mg++ supplement to avoid the twitching and to keep your blood ph in tact. The glycerin is taking this one step further though, same principle though. Our bodies are reacting the same way as someones body would react after serious excersise, this is not deadly but you are getting mineral depleted. If any of you where wondering yes I also have the left eye twitch

I would not worry about this at all, the only time you need to worry is if you are experiencing paplipations, but this would most probably be the nicotine, but if you are smoking nicotine free propylene glycol and you get palps it means your mineral balance is completely messed up.
it would be nice if someone who vapes could go for a blood ph test to confirm my theory, im 98% sure this is what is happening thoug....
take care!