automation to calculate date task due is not working for some tasks

I have a Board with a list of tasks and an automation set up to popular a column “Scheduled Task Date” which is calculated based on the “Project End date”, and the “weeks from the end date” to calculate the scheduled task date and format it as a date. Of the 13 tasks it has calculated them all correctly, with the exception of two. When I have 4 weeks from 26 December, for example, it is deducting 4 weeks rather than adding them on. (I have other tasks where I have put -4 and it calculates correctly 28 November but it is also calculating the same date for adding 4 weeks on"). I hope this makes sense and would appreciate some help!