Time Intelligence - Last Date Overdue

I see that you said as new to Power BI, sharing this tip: Column value is persisted when you refresh the data. whereas Measure is calculated as and when needed.

I am providing the column syntax for you: (little bit detailed steps for your understanding)

Next Refill = 

var _Cust = 'Table'[Customer]
var _DateDisp = 'Table'[Date Dispensed] 

var _MaxDate = CALCULATE( Max('Table'[Date Dispensed]), FILTER( all('Table'), 'Table'[Customer] = _Cust)) 
var _SameMaxDateRow = IF ( _DateDisp = _MaxDate, True, False)

Return IF( _SameMaxDateRow , _MaxDate + 'Table'[Days], BLANK())

Overdue = 
 var _bufferDt = 'Table'[Next Refill] 
 var _IsOverdue = IF ( _bufferDt >= TODAY() + 15, "FALSE", "TRUE" )
 
 Return IF (  IsBlank('Table'[Next Refill]), BLANK(), _IsOverdue)

FYI: Overdue has to be data type as text. If the data type is True/false, then all the blank values become false. 

Hope this helps!