Value not returning all selected in DAX formula

I'm hoping someone sees something obvious that I'm just missing here. I have a formula that I want to use to create a column that gives the total from the prior inception date year. I have a slicer in my report and when nothing is selected, it works as expected and adds all the prior years, subtracts the current year value and gives a total. Great, that's perfect. If I select a single year, it gives me exactly what I need. The issue comes when I try selecting multiple years. It seems to only be pulling some of the selections, and leaving one out, so the total is off just a wee bit. For example, if I select years 2022, 2021, 2019 then it excludes the prioryearGWP for 2019 from the total. I'm not sure why this is occurring. I think my Var Period is where my issue lies, but I'm fresh out of ideas on what's missing.

Here's my dax:

Prior Year(s) Gross Written Premium = 
VAR SelectedYear = 
SELECTEDVALUE('Table'[Inception Date].[Year] )
VAR CurrentYear = 
MAX('Table'[Inception Date].[Year])
VAR PolNumCacTable = 
CALCULATETABLE('Table', ALLEXCEPT('Table','Table'[Policy Number],'Table'[Agreement ID]))
VAR PriorYear = 
MAXX (FILTER ( PolNumTable, 'Table'[Inception Date].[Year] = (CurrentYear - 1 )), 'Table'[Inception Date].[Year] )
VAR PriorYearGWP = 
SUMX ( FILTER(PolNumCacTable, 'Table'[Inception Date].[Year] = PriorYear), 'Table'[Gross Written Premium Amount] )
VAR CurrentYrGWP = 
SUMX ( FILTER(PolNumTable, 'Table'[Inception Date].[Year] = CurrentYear), 'Table'[Gross Written Premium Amount] )
VAR Period = 
IF (
SelectedYear = (PriorYear + 1),
CALCULATE(PriorYearGWP, ALLSELECTED('Table'[Inception Date].[Year])),
CALCULATE(SUMX('Table', [Gross Written Premium Amount]) - CurrentYrGWP)
)

RETURN
{Period}