Hello ! No it's not normal and there is no mutation neither , your plant have been stressed because your light schedule must not be a real 12/12. What did you change since week 16 ? Is that happening with every plants or just one ? If it's just one then cut her , and try to find a light leak ( the "airflow holes" , a box half open during the night, 2 or 3h of light in the middle of the night due to a mistakes on the timmer ?) . If you can find the difference between week 16 and now you will find what cause your issue. Hope it will help you 😁👍