Use latest date available if selected date doesn't have data

Hi elmurat 

If i understand you right , first make sure your slicers is single-select and make the measure fall back to the latest date on or before the selection for the current LoanID.
First : Read the slicers
Selected Year-End Date =MAX ( YearEnd[YearEndDate] )

Selected Month-End Date =MAX ( MonthEnd[MonthEndDate] )

2nd: “Last available date ≤ selection” (keeps LoanID filter)
Last Available Year-End Date =
VAR sel = [Selected Year-End Date]
VAR dWithinOrBeforeSel =
CALCULATE (
MAX ( YearEnd[YearEndDate] ),
ALL ( YearEnd[YearEndDate] ),
YearEnd[YearEndDate] <= sel
)
VAR dFallbackOverallMax =
CALCULATE ( MAX ( YearEnd[YearEndDate] ), ALL ( YearEnd[YearEndDate] ) )
RETURN COALESCE ( dWithinOrBeforeSel, dFallbackOverallMax )

Last Available Month-End Date =
VAR sel = [Selected Month-End Date]
VAR dWithinOrBeforeSel =
CALCULATE (
MAX ( MonthEnd[MonthEndDate] ),
ALL ( MonthEnd[MonthEndDate] ),
MonthEnd[MonthEndDate] <= sel
)
VAR dFallbackOverallMax =
CALCULATE ( MAX ( MonthEnd[MonthEndDate] ), ALL ( MonthEnd[MonthEndDate] ) )
RETURN COALESCE ( dWithinOrBeforeSel, dFallbackOverallMax )

3rd : Principal “as-of” each side
Principal @ Year-End (as-of) =
VAR d = [Last Available Year-End Date]
RETURN
CALCULATE (
MAX ( YearEnd[Principal] ),
KEEPFILTERS ( YearEnd[YearEndDate] = d )
)

Principal @ Month-End (as-of) =
VAR d = [Last Available Month-End Date]
RETURN
CALCULATE (
MAX ( MonthEnd[Principal] ),
KEEPFILTERS ( MonthEnd[MonthEndDate] = d )
)

4th: Final difference
Principal Diff (MonthEnd – YearEnd) =
VAR m = [Principal @ Month-End (as-of)]
VAR y = [Principal @ Year-End (as-of)]
RETURN IF ( NOT ISBLANK(m) && NOT ISBLANK(y), m - y )

Same idea mentioned in this problem : 
https://community.fabric.microsoft.com/t5/Power-Query/Find-Most-Recent-Date-Relative-to-another-Date-Column-in-Power/m-p/4790379 

If this helps please mark my answer as Solution and drop a Kudos 🙏