As per the request of some community members I am cross posting my research on Gantt chart issues in the Project Management module for ERPNext 12 on this forum. Hopefully I can gather more input about testing procedures here!
"When moving tasks using the Gantt view the task will have its’ date set to 1 month ago and task duration to set to 1 day. If the task has tasks dependant on it it will also recursively reset the linked tasks. This is both in the Gantt view and visible on the the task properties itself.
Please see order of actions below with screenshots. (Movement was performed from current day to next calendar day by dragging the centre of the task)."
Screenshots and further information can be accessed via the github link below.