Cassidy,
I think you're using the wrong field in your 2nd line.
If(IsNull([Lead Time +Transit Time (Weeks)]),null,
If(IsNull([PO Issue Date]), WeekdayAdd([
Plan Issue PO Date], [Lead Time +Transit Time (Weeks)]*7),
WeekdayAdd([PO Issue Date], [Lead Time +Transit Time (Weeks)]*7)))
The 2nd line was saying "if it is empty" then use that same date (empty) and add Weeks * 7 days to it - which would be wrong.
OR you could have written it as:
If(IsNull([Lead Time +Transit Time (Weeks)]),null,
If(
not IsNull([PO Issue Date]), WeekdayAdd([PO Issue Date], [Lead Time +Transit Time (Weeks)]*7),
WeekdayAdd([Plan Issue Date], [Lead Time +Transit Time (Weeks)]*7)))